When Richard Griffiths's Martin Dysart admits "I have never been on a horse in my life", the understandable reaction of his audience is to wonder whether he really needs to point this out. For Griffiths brings to the part - as he must - a remarkable embonpoint that ought certainly to preclude any form of equitation, at least if the horse's comfort and safety are to be considered. Comparing his rolling bulk with the trim physique of his co-star in Equus, the 8st Daniel Radcliffe, one concludes that here is a double act that could easily borrow the name Little and Large.

Would there be a similar contrast in the scale of the acting skills on display? This seemed possible when news of the pairing was broadcast - the hugely experienced Griffiths being widely recognised as one of the subtlest actors of his generation, while Radcliffe is known only (only!) for his screen portrayal of the schoolboy magician Harry Potter. In fact, it becomes apparent very early in director Thea Sharrock's gripping revival of Peter Shaffer's 1973 success that there is no gross imbalance in the talents on display. In the challenging, frightening arena of live theatre, Radcliffe proves he can truly do the business.

It greatly assists his performance, perhaps, that at 17 he is exactly the age of the character he plays. In presenting the tortured Alan Strang, who has blinded six horses in a frenzied attack at the stables where he works, he first shows us a traditional troubled teenager - sullen, uncooperative, uncommunicative, except when it comes to the spouting of TV commercials for Milky Bars and Martini. Then, as psychiatrist Dysart probes deeper, we come to experience with him - through Shaffer's clever weaving of the past with the present - something of the dangerous passion that he feels for horses, beasts presented to our view in all their magic and magnificence by dancer Will Kemp and five other performers tricked out (designer John Napier) with wire heads and hooves.

The trouble arises from the impact on Strang of the deep religiosity of his hellfire-spouting mother (excellently played by Gabrielle Reidy) becoming confusingly overlaid in his mind with images of horses and memories of the excitement stirred by childhood encounters with them. A key moment comes when his grumpy atheist dad (Jonathan Cullen) replaces a Crucifixion scene on the boy's bedroom wall with an iconic portrait of a horse. The result for Strang is a new religion all of his own, with 'equus' the object of worship.

Disaster happens, though, when these venerated beings witness his failure in a first attempt at sex with raunchy stable girl Jill (Joanna Christie). This 'in-the-buff' scene has, not surprisingly, been the focus of publicity concerning the production, but it seems entirely natural, and by no means shocking, in the context of the action.

If the plot sounds a little unlikely - and, indeed, a little dated in Shaffer's espousal of the discredited theories of R.D.Laing - it is certainly the case that plausibility is lent by Griffiths's superb work as Dysart. His laid-back, humorous, self-deprecating presentation of the worried shrink - in whom there is definite envy for the vivid experiences of his patient - is a far cry from the portentous posturing provided by Richard Burton in the 1977 film version of the play. His performance should not be missed.

Equus continues at the Gielgud Theatre until June 9 (box office: 0870 950 0915).