When Metro passengers began stealing the Dupont Circle posters out of the subway trains, artists David Toerge and Tom Engeman knew they were on to something.

The posters were supposed to provide a month-long guide to the circle's merchants, but within days after they were put up in the subways, the posters were gone.

"Metro people called us up and asked for more posters," explained Engeman, a former Washingtonian magazine art director. "We figured that if people were going to steal them, we ought to print some more and sell them."

The second press run quickly sold out, putting the Gyro Art Poster Co. into business as Washington's newest and smallest advertising service.

Photographer Toerge and artist Engeman said they are on their way to creating a monthly medium that enables small businesses to advertise economically, and simultaneously improves the decor of the subway trains -- as well as the homes of poster thieves.

The 1,200 copies of the first Dupont Circle poster were followed by a second edition last month. A third Dupont poster is planned, and a Farragut Square version has been sold and stolen just as quickly as the original Gyro product.

Toerge and Engeman now are tackling the insomniac market with an "all night long" poster that will promote around-the-clock businesses -- all-night diners, drugstores, supermarkets and emergency services.

Their goal is to sign up 100 advertisers at $250 apiece and expand distribution throughout the Washington area, then put up a new poster each month on the Metro trains and in the windows of advertising merchants.

Engeman said he and Toerge know the posters won't stay put. Last month Engeman was in the Los Angeles airport when a women got off a plane carrying one of his Farragut Square posters. He said he was so happy to see his artwork so far from home that he didn't ask her whether she'd paid for it.