WITH ITS IMPRESSIVE OLD MASTERS, life-size marble sculpture and skillful use of lighting, art dealer Ramon Osuna's Northwest town house looks like someone's fantasy of a 19th-century European museum. In fact, it is intensely practical and designed to change at a moment's notice.

"You can change from one decor to another in four or five hours," says Osuna, who collaborated with interior designer Antony Childs to come up with the basic scheme for the five-story house three years ago. Paintings are hung on bronze chains for quick removal (and fewer nail holes), walls in the "public" rooms are painted elephant gray (a neutral background that accommodates a wide variety of art) and wood floors are kept predominantly bare (so sculptures can be moved easily).

In fact, since Osuna uses his residence to showcase Old Masters and 19th-century paintings and furniture (20th-century works make up the bulk of his downtown gallery), the painting you admired in the foyer last week may be gone by next. Art dealers can't resist a revolving inventory.

Lighting is key to the quick-change atmosphere. A museum-quality system of track lights, designed by Gordon Anson of the National Gallery of Art, targets the prominent works of art; abundant candles illuminate the decorative objects in intimate detail.

"There's a lot of chiaroscuro," says Childs. That may be a painting term, but Childs has a point: By anyone's standard, the play of light and dark is certainly dramatic, more fantasy than reality. But for Osuna, whose exposure to fine works of art dates to his childhood in Havana, living in an always-evolving, fantasy-like environment is purely natural.

"I think of it as reality," he says matter-of-factly.