Where is Washington's cheapest gasoline? That is what many area motorists want to know now that gasoline prices, after falling to an unusually low level in early February, are starting to rise. The question will gain importance as the weather warms, gasoline demand increases and prices begin their seasonal rise.

But it is possible to keep gasoline bills to a minimum even during a time of peak demand and peak prices if you know where the bargain stations are located. These stations may be branded or unbranded -- a distinction that has come to have less meaning in recent years with the disclosure that major oil companies often swap fuel, usually to save transportation costs, and that gasoline sold under one brand actually may have been refined by another.

In the most recent survey by the American Automobile Association's Potomac division, the price for self-service cash regular leaded now ranges from 99 cents to $1.109 a gallon, the price for regular unleaded from $1.069 to $1.279 a gallon and the price for super unleaded from $1.149 to $1.399 a gallon.

Based on previous pricing practices, the lowest prices typically are found in Prince George's County. The highest usually are in the District of Columbia. Between the two extremes are Montgomery County and Northern Virginia.

Two sections of the Washington area that are noted for their low prices, due to scrappy neighborhood competition, are:

* Rte. 1 (Baltimore Avenue) in Prince George's County, from the University of Maryland to the Beltway. This mile-long strip, commonly known as Gasoline Alley, is lined with stations such as Homoco, 8616 Baltimore Ave., where the self-service cash price yesterday for regular leaded was $1.019 a gallon, regular unleaded was $1.079 a gallon and super unleaded was $1.209 a gallon. The Shell station at 9322 Baltimore Ave. had these self-service cash prices: regular leaded for $1.029 a gallon, regular unleaded for $1.099 a gallon and premium unleaded for $1.279 a gallon.

* Rte. 7 (Leesburg Pike) in Fairfax County, particularly in the area near the Beltway. One of the lowest-priced stations in this commercial stretch is the Scot station, 8411 Leesburg Pike, Tyson's Corner, where the self-service cash regular leaded yesterday was $.999 a gallon, regular unleaded was $1.069 a gallon and premium unleaded was $1.199 a gallon.

There are other pockets of competition in the Washington area with low prices. The area along Rte. 50, from Seven Corners to the Beltway, and the Marlow Heights area both tend to have below-average prices.

In addition, there are a number of stations that traditionally charge prices that are lower than the area average. Examples, which generally are based on the price for self-service cash transactions, include:

* Crown, 5500 Franconia Rd., Alexandria. Regular leaded yesterday was $1.019; regular unleaded, $1.069, and premium unleaded, $1.199.

* Cloverly Shell, 15541 New Hampshire Ave., Silver Spring. Regular leaded $1.019; regular unleaded, $1.119 and super unleaded, $1.219.

* Burch's Gulf, 400 W. Broad St., Falls Church. Regular leaded $.999; regular unleaded, $1.119, and super unleaded, $1.279.

* Arco, Kimball's Service Center, 2 Ashton Rd., Montgomery County. Regular leaded, $.999; regular unleaded, $1.119 and super unleaded $1.219.

* Merit, New Jersey and Rhode Island avenues NW. Regular leaded $1.029; regular unleaded $1.109, and premium unleaded $1.219.

* St. Barnabas Exxon, 5622 St. Barnabas Rd., Oxon Hill. Regular leaded, $1.039; regular unleaded, $1.109, and super unleaded, $1.259.