Never in modern times has such excitement been stirred by the prospect of viewing a very few inches of adolescent male flesh. The flesh in question was duly exposed last night when Daniel Radcliffe, fresh from fame as Harry Potter, bared his body as 17-year-old horse destroyer Alan Strang.

Peter Shaffer's dated, though still gripping psychological drama Equus broods about the dangers of surrendering both to instinctual passion and rationalist conformity.

Radcliffe plays Strang and, glaring eyes apart, emphasises his vulnerability. This lower-middle class boy blinds six horses and then refuses to speak about his ghastly crime.

When it came down to basics, though, Radcliffe's striptease with his would-be girlfriend, Jill, an equally nude Joanna Christie, loomed small compared with the impact of the stylised scene of the horses' destruction.

Six actors wear wire-frame horses' heads, with metal platform hooves. Their hollow eyes glow in the dark. Whinnying, rearing and hoof-stamping, they suffer Alan's attacks.

For sheer shock effect nothing else in Thea Sharrock's undercharged production matches this scene - but it can.

For Equus launches an intriguing why-did-he-do-it investigation. Shaffer tuned in to an early Seventies reverence for psychiatry and psychotherapy as systems to probe and relieve the workings of the mind.

John Napier, who designed the original 1973 National Theatre production, now sets the play expressionistically in the dark night of the soul.

Three or four cube shapes serve as chairs centre-stage while the stables for horses encircle the action. All the exits are sinister, long walkways.

Richard Griffiths is far too affable, winsome and relaxed as the psychiatrist Dysart.

Alan's parents, Jenny Agutter's declamatory, stilted magistrate, and the horses' owner all come to tell stories that offer clues and insights into Alan's mental state.

The boy himself, played by Radcliffe with an air of pathetic, small-scale belligerence, becomes voluble thanks to Dysart's unbelievably successful promptings.

We the audience are, therefore-offered tempting opportunities of psychological detective work to work out what has gone wrong. Unfortunately, Strang's motives remain enigmatic and ultimately unbelievable pretexts for butchery.

Son of sexually repressed parents - a religious mother and an atheist father - Alan discovers early liberation and sexual excitement astride a horse ridden by a handsome young man.

His bedroom picture of sado-masochistic images of Jesus is replaced by one of a horse, the animal that he invests with both divine and erotic salience.

His reason for blinding the horses may have to do with revenge for the fact that his desire for them renders him impotent with Jill or shame that these divine creatures should witness his sexual antics.

Shaffer's casebook finally leaves you shaking your head in disbelief.

Of equal concern is Dysart's eloquent contempt for his own sexless, empty life and fear that in curing Alan - an unbelievable process - he leaves him to joyless conventionality. This smacks of romantic tosh: passion is no inherent good and Alan's eros made him a social danger.

Equus works as theatre, if not as psychological case-study, if shrink and patient are made desperate by the battle over passion, rationalism and religion.

Griffiths, though, packs no emotional punch, lacking the agitated despair Alec McCowen brought to the role, while Radcliffe's touching, little-boy lost Alan never convinces you he is wild with desire for horses or girls. Equus still fascinates, but this revival lacks horse-power.