NO matter how often you are warned in advance, the small size of Leonardo da Vinci's Mona Lisa at the Louvre in Paris still surprises you.

Likewise, no matter how many T-shirts and tea towels, mugs and jigsaws, badges and posters are mass-produced by National Gallery, nothing can prepare you for the scale of the most famous equine painting in Britain, George Stubbs' Whistlejacket.

History states that the 2nd Marquess of Rockingham's most prized racehorse responded to his first sighting of the life-size oil painting by attacking it, after mistaking it for another horse. It is not recorded, however, whether Stubbs changed the horse's eyes to a more fiery lustre as a result, nor how Whistlejacket would have reacted if Rockingham had decided that George III should be pictured astride the horse, as first planned. Instead Whistlejacket, his mane wild, his eyes wilder still, stands proud without a saddle, hooves rearing up into the air, with no distracting background (or regal rider). Rockingham's aesthetic reasoning was impeccable.

On seeing the 11ft-high portrait on its first ever outing at York Art Gallery, you are more likely to step back than launch an attack, such is the presence of a 1762 painting that dominates the exhibition Stubbs And Whistlejacket In York.

Enter the white-walled gallery, and Whistlejacket stands out on the right-hand side, and not only because of his being mounted on a blue rectangle. It is a huge picture, so huge and heavy that the floors needed strengthening with special boards, and the gallery doors had to be taken off temporarily in order to accommodate the painting's admission.

If you live in York and never bother to enter the wondrous Minster, please don't let the visit of Whistlejacket pass you by between now and August 31 (when Whistlejacket's grand tour will take him onwards to Leeds). Stubbs's masterpiece is, unquestionably, the finest painting associated with our city, painted by the finest equine artist who studied human anatomy here in the 1740s and married here too.

Whistlejacket has his place in York's history. Foaled in 1749 and possibly named after the medicinal drink of gin and treacle that matched the golden tone of his coat, Whistlejacket recorded his most famous victory on York's Knavesmire in 1759, where he won 2,000 guineas for Rockingham, who promptly retired him to stud. (Nothing changes in the world of horseracing, it would seem.) On show alongside the Rockingham racing silks of plain green, on loan from York Racecourse, is W Pick of York's racing calendar from 1709 to 1785, open at August 21 1759, the day of Whistlejacket's victory over Mr Turner's Brutus in an "exceeding fine heat, being strongly contested the whole four miles and won by a length only".

Whistlejacket may dominate the exhibition, but not exclusively so. Loans from the National Gallery and Royal Academy of Art and York Racecourse complement artefacts from York Art Gallery's own collection in an exhibition divided into four. Scenes of York in Stubbs's era and a Stubbs self-portrait at the age of 57 on hard-wearing enamel (inspired in 1781 by his friend Josiah Wedgwood) stand to the left, followed by equine works, some by Stubbs's son George Townly Stubbs. On the back wall is Stubbs senior's 1767 study of Gimcrack, the horse that gave its name to the Gimcrack Club, Gimcrack Stakes and a now-closed pub in Fulford Road, despite never winning a race at York. A stealthy pointer and a spaniel on the scent affirm Stubbs's skill at canine portraiture too.

For all his sublime artwork, Stubbs's working methods were stained by controversy, a stain that survives to feature in Charles Oakley's 1989 trompe l'oeil, a wooden box in mixed media entitled Stubbs No 2. Oakley conveys the contradictions between Stubbs's scientific and bloody research - conducted in secret at a rented farmhouse in Horstow, near Hull from 1756 to 1758 - and the romantic, untamed nature of his equine art by showing Stubbs paintings and anatomical drawings hanging from meat hooks, a gruesome image indeed.

"He understands anatomy better than any of the ancients," says the writing above 12 of Stubbs's studies for the 1766 book The Anatomy Of The Horse and The Anatomical Or Skeleton Tables.

"He'll plunge his knife and thrust his fist in," it continues, and he did more than that in his somewhat grim business of dissecting horses. He bled them and injected their veins with talon in order to maintain their shape. Carcasses were positioned in walking poses and held in place with straps and pulleys, whereupon Stubbs would take notes and sketches at all angles. Science and art duly come together in Whistlejacket.

Before leaving the exhibition, make sure to see the pictorial account of the race between Mr Flint's Volunteer and Mrs Alicia Thornton's Vingarillo, when only the slip of her girdle after three miles denied the side saddle-riding Mrs Thornton victory in front of 100,000 people at York Racecourse.

One comment from the crowd in S W Fores's Too Far North For The Knowing Ones has echoes of southern racegoers' decision to turn their nose up at Royal Ascot at York in June 2005. "I'll take care they shall never catch me so far north again," it reads.

Their loss, as always.