FILM / The horror that dare not come of age: The Exorcist is 21 years old this year and still as shocking as ever. John Lyttle reports

Saturday, nudging the witching hour. The packed audience at London's MGM Trocadero is palpably tense while the scratched print unspools, as it does every Saturday, a regular midnight mass.

There's Linda Blair as Regan, the movie star's daughter, tossed on her bouncing bed, like a little girl getting her birthday 'bumps'. Regan spews obscenities as she mutilates herself with a crucifix, forcing her mother to lick the blood. She vomits pea-green jets. She levitates, demonstrates telekinetic powers, claims to be Satan himself. Her limbs erupt in sores and wounds. Finally, through the pale, tortured skin, comes an unexpected and pitiful plea: 'Help me.'

The Exorcist is 21 this year. If you're in your mid-thirties, you are unlikely to have forgotten the first time you clapped eyes on William Friedkin's movie of William Peter Blatty's bestseller (60 weeks in the national book charts). It has been endlessly imitated (Beyond the Door, The Manitou, Abby), parodied (Hysterical, Repossessed) and plagiarised (in every movie with gory prosthetic special-effects since). But when The Exorcist was released on Boxing Day 1973 no one had ever seen anything like it. People snaked round the block for their chance to throw up, faint and stagger for the exits.

The Exorcist spoke to the turbulent times: it was hard-core graphic, as casually unsparing and guiltily exciting as the porn that was then sneaking out of back-street cinemas into respectable first-run houses (Deep Throat packed them in during 1972) and as violent as the nightly news from Vietnam. True, there had been shockers before - the crude drive-in canon of Herschell Gordon Lewis (Two Thousand Maniacs] Blood Feast) and Romero's Night of the Living Dead, but they were cheap, independently-hewn nightmares. The Exorcist was different: big-budget (an astonishing dollars 10m), mainstream and Hollywood.

That was the real shock. Not so much the idea of a child suffering the torments of the damned. Nor even the sight of it - Dick Smith advanced the art of FX light- years to strip the flesh off Linda Blair's legs and swivel her head a full 360 degrees. It was simply the fact that Warner Brothers, a major studio, thought this a subject fit for 'family entertainment'.

And incredibly, on its release, The Exorcist was awarded an American R, not an X: a night out for all the family. As Pauline Kael noted, if the film had cost under a million or had been made abroad, the ratings board wouldn't have dared. But Warners needed to protect its investment and that meant attracting the widest possible audience: The Exorcist swiftly became (and still remains) the biggest-grossing horror film of all time.

Audiences turned out in record numbers because The Exorcist, despite its bad language, blasphemy and clinical brutality, was sold as an old-fashioned clash of Good vs Evil. (The director Paul Schrader once called the film's climax 'cinema's ultimate confrontation, God against the Devil').

The movie is painfully moralistic, in a back-to-basics kind of way. The Horned One picks the MacNeil residence because it's a broken home, headed by that other recently demonised figure, a single parent. Yet The Exorcist is less an 'apostolic work' (Blatty's claim) than a reactionary commentary on the generation gap, a backlash against the youth explosion of the Sixties. (It is one of the movie's ironies that its visceral assaults would have been impossible without the inroads made into 'good taste' during that progressive decade.)

Regan is clearly identified as being a few days short of 13, that difficult age when children begin to rebel. There's an even bigger tip-off: Chris MacNeil, Regan's mother (Ellen Burstyn), is in Georgetown, Washington, to star in Crash Course, a celluloid tale of campus revolt. The scene we see being shot before Regan goes seriously off the rails is of a student near-riot, a snapshot of a period undergoing massive social upheaval. MacNeil can handle the extras stampeding the university library, but Regan proves uncontrollable. As Stephen King remarks in his horror study, Danse Macabre, The Exorcist can be read as a 'movie for all those parents who felt that they were losing their children and could not understand why or how it was happening'.

The Exorcist actually pre-dates Mommie Dearest as a satire of mother-daughter conflict. Burstyn's control-freak mode (she's always barking orders) is short-circuited by 'the thing upstairs'; Regan begins her destruction of tedious middle-class mores by crashing Mum's cocktail party and peeing on the carpet. And Regan is a horny devil, blatantly sexual in a manner that Mum, a recent divorcee, can't allow herself. Implicitly, The Exorcist views those sexual impulses as disgusting. The movie's motif turns out not to be Good vs Evil but control: control of youth, womankind, female libido.

Regan may display classic symptoms of anorexia (vomiting, drastic weight-loss, self-mutilation) but she's also displaying absolute command over her own body. And, naturally, the movie's menfolk fight this heresy; the film is about possession in more senses than one. Doctors penetrate Regan with needles, psychiatrists penetrate her psyche and Father Karras (Jason Miller) and Father Merrin (Max von Sydow) want to penetrate her very soul. Small wonder Regan is chastised for her libidinous exhibitions. The fusion of sex and punishment routinely leads reference books to mislabel the genital mutilation scene 'the masturbation sequence'. This poor child's body is deemed so powerful that finally it must be bound, hand and foot; a pose queasily reminiscent of Catholic iconography and S & M fantasy.

The British Board of Film Classification banned The Exorcist on home video at the height of the first Video Nasties scare in the mid-1980s, citing its influence on 'impressionable minds'. Rather than set about disproving the increased incidence of personality disorders in those countries where the video was freely available, Warners kept quiet and accepted the ban. It nearly paid off. Four years ago it looked as if the BBFC's sanctions would be lifted. Then the storm over 'ritual satanic abuse' broke. Warners shelved its plans. The video release remains in limbo.

Which proves that, 21 years on, The Exorcist still has the power to disturb, if only because what it showed in the permissive Seventies would be impossible in the censorious Nineties, newly obsessed with childhood innocence (and corruption) in the wake of the Jamie Bulger case. As The Exorcist's tatty descendants (the ironically titled Child's Play series in particular) are predictably rounded upon by those determined to establish a link between horror fiction and moral lassitude, the movie that started it all lives an alluring half-life; momentously influential, blatantly sensational, now hidden by the shadows. There's nothing quite like the lure of the forbidden. Which is why the faithful turn up every Saturday to worship at the celluloid shrine and listen to little Regan scream.

THE CRITICS: WHAT THEY SAID THEN AND NOW

'My first reaction was one of irritation and then anger that a film-maker of Friedkin's resource should lend his talent, and that of a good cast, to something so essentially meretricious . . . If the film's a religious experience, I'm glad I'm agnostic.' Derek Malcolm, Guardian, 14/3/74

'I thought it a good piece of genre film-making and a completely cynical exercise in money-making. The idea that it had a message is laughable: a message to their bank managers, perhaps. I did find it fairly disgusting - physically, not morally. The use of the little girl is pure Hollywood.' DM, today

'The Devil and all his special-effects are the things the people flock to see. They're well worth the money, most of them. The Exorcist is a very well made horror film with pretensions to something better.' Alexander Walker, Evening Standard, 14/3/74

'I haven't changed my mind. It's merely a superior shocker, not an 'apostolic work'. I think it's about its times in the sense that we'd had Rosemary's Baby and Time magazine asking if God was dead. I can't understand why it isn't available on video, unless the British Board of Film Classification thinks it's expelling satanic forces by not allowing its release.' AW, today

(Photographs omitted)