To some, these historical references will come as a surprise. They must be a relief to the National Gallery trustees and to the three brothers from the Sainsbury supermarket family - Sir John, Timothy, and Simon - who have personally pledged the money for the building of the extension. After all, the new building does have to fit alongside William Wilkins's neo-classical exercise of 1838 and the planners might object to anything too wacky.

Venturi's practice, Venturi, Rauch and Scott Brown, includes the name of his British-born wife, Denise Scott Brown. Denise, herself a formidable, and somewhat eccentric academic, is aggressive, ambitious and voluble. Bob sits quietly in the background, chipping in with the odd word. There is no doubt that Bob is in charge of the office; equally, there's the feeling that his emergence from academic obscurity to the architectural big time is due not only to his un- doubted talent but also to the energy of his wife.

The two tend to work together, leaving the business side to the third senior partner, John Rauch. Venturi's architects feel like apprentices to a master craftsman: he asks to see every drawing produced and normally makes some small personal amendment. His designs for furniture and tableware are as painstakingly created as those for his buildings.

Venturi works for fun - he is always in the office on a Saturday, and often on a Sunday too. Otherwise, he is to be found taking his son to fun parks, tinkering with his marvellous, genuine art nouveau house, or travelling, sketchbook in hand,particularly round Europe.

Unlike other American architects who are currently invading London in search of rich City commissions, he is not wedded to commercialism. He does not spend his life flinging up decorated sky-scrapers like his flamboyant compatriot Philip Johnson, who is designing a "gothic" office complex next to Tower Bridge.

Venturi has in truth built only a handful of projects in the 30 years he has been practising, and has never built anything in Britain or indeed, outside America. Yet the fact that architects on both sides of the Atlantic have in recent years moved away from anodyne glass boxes to a richer, more ornamental vocabulary owes a great deal to the pervasiye influence of Venturi. He is regarded with affection as a slightly oddball mentor.

The National Gallery commission came at just the right moment for Venturi. This father of post-modernism seemed at one stage to depend almost entirely on house commissions from wealthy friends and relatives to supplement his university teaching. By the 1980s, he was seeing his protégées start to get famous, yet never had a major project himself that was right in the public eye. He did not conceal his delight when he landed the National Gallery job. "He was overjoyed by the thrill of getting what he saw as the reward of 30 years", says a former colleague.

Venturi believes firmly in diversity. Just because you've designed a building in one style doesn't mean that all your others have to be like that, he says. He embraces symbolism and ornament and rejects the old modern movement dictum that form follows function. "This should no longer be the case," he says in a recent, typically scholarly treatise. "We now look for inspiration to an architect like Sir Edwin Lutyens who, especially in his domestic architecture, worked in a variety of historical and decorative styles."