All the pre-show hype, predictably, was about 'Harry Potter's nude scene' but Equus is a play that strips actors to their innards. Audiences do not escape lightly either.

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So, yes, it can be reported faithfully that on Shaftesbury Avenue at halfpast nine last night the star of Hogwarts, Daniel Radcliffe, removed every last thread of his kit.

So did a pretty young actress (Joanna Christie) and the two of them had a prolonged fumble. But that is less than half the story.

Such is the force of this troubling, turbulent play that the nudity brought no frisson.

There was no great moment of voyeuristic titillation. What was striking, instead, was the emergence of young Dan Radcliffe in the artistic raw, tested as an actor and found equal to a stretching role.

The fashionable, 30-something women hanging over the edge of the Gielgud Theatre's boxes may have been salivating beforehand like Hogarthian trollops. Afterwards they were surely left poleaxed by a great drama done well.

He has put Harry Potter tidily behind him - should he so desire. You could say, if you still had any inclination to equine images, that he has leaped from a tiring mount on to a fresh, frisky charger with a circus rider's aplomb.

Equus is dark, adult, very difficult. Mr Radcliffe plays Alan, 17-year-old son of a strict, atheist father (Jonathan Cullen) and a soft, churchy mother (Gabrielle Reidy). Something

of a test case for Philip Larkin's theory about what parents can do to you, Alan has started to confuse worship of Christ with physical adoration of horses.

Yet after a moment of sexual embarrassment he succumbs to an almost balletic dance of madness and gouges the eyes from six poor beasts. Their neighs will make your spine shiver.

The play opens with him being admitted to a psychiatric hospital. At first he will only sing jingles from TV adverts.

His doctor is Martin Dysart (Richard Griffiths).

Peter Shaffer's skill as a playwright is to meld the development of the doctorpatient relationship with an exposition of the violent unhappiness which brought Alan to this confinement.

Mr Griffiths is the true star of the show. It is always slightly surprising that so bulky a man can prove so delicate a performer, so subtle with a gesture of the palms here, a check in the voice there.

He is magnificently relaxed on stage. Daniel Radcliffe could not hope for a greater master to watch.

He should learn from the way Mr Griffiths gives cues some air, the way he varies the shape of his mouth, the tilt of his head, the tension in his shoulders.

Alan is a plain Hampshire boy, though. Mr Radcliffe is right not to lend him too much sophistication.

He gives him a faint West Country burr and a teenagerish glower. Even when he is reduced to his muscular pelt, a fuzz of Velcro hair descending from his navel, it is the nakedness of Alan's mental disarray that hits you.

Thea Sharrock's excellent, brisk production is helped by contrasts of black, minimalist set against shafts of white side lighting.

The horses are created memorably with metal masks and hooves that clop frantically when exposed to their not so stable lad.