For the first time in nearly a year, the lights are burning brightly at night in Washington area service stations, as dealers expand their hours in an effort to sell unusually high supplies of gasoline.

The stocks of gasoline, heating and crude oil now are the highest in history here and throught the nation. So are the prices -- $1.27 1/2 a gallon in Washington, on average.

Within the past few days, five oil companies that supply 73 percent of the Washington area's gasoline -- Amoco, Exxon, Gulf, Sun and Texaco -- have announced May supply increases for their stations.

Barring an import cutoff, oil executives predict adequate supplies -- no gasoline lines -- throughout the spring and summer. Prices could continue to rise sharply, especially around May 15, when a 10-cents-a-gallon "conservation fee" imposed by President Carter begins to take effect.

"We're back to normal, seven days a week," Forrest Pomeroy, an Amoco dealer on River Road NW, said yesterday. Until recently, his station was open only until 3 p.m. weekdays and closed weekends because he could sell all of his available gasoline during those hours.

Now that his Amoco representative has told him he can have virtually all the gasoline he wants, Pomeroy plans to stay open weekends and until 10 p.m. weekdays to compete with nearby Shell and Mobil stations that are open at night.

If that doesn't work, and he still has excess gasoline, Pomeroy may be forced to trim his profit margins a little to compete as some dealers nationwide have reluctantly begun to do.

"Some retailers have actually moved their prices down a few cents, and some absorbed modest increases from their suppliers without passing them through [to motorists]," said Joe Strain, a Sunoco analyst.

Mark Emond, editor of the authoritative Lundberg Letter, a petroleum marketing publication, said that America may see "a summer of eroding [gasoline] prices."

That probably will not mean that prices go down, but that they will rise less steeply than they did in the period from last fall through early this spring, when gasoline prices were going up six to seven cents a gallon each month.

That trend halted in April, according to a Washington Post survey of local stations. Between April 2 and 24, the average price of a gallon of gasoline in the Washington area increased by only two-tenths of a cent, to $1.27 1/2 for all grades. Unleaded regular was an average of $1.29.4, while leaded regular was $1.25.6.

The Post survey showed that prices went down at several stations, and that the average price for one grade -- self-serve regular -- declined two-tenths of a cent during the period.

One reason prices are softening, the oil executives say, is because there is no shortage on world markets. Another key reason is that motorists are conserving.

In the Washington area and nationally, people are using 8-to-10 percent less gasoline than they were a year ago, even though a year ago supplies were already being somewhat curtailed at the beginning of Memorial Day weekend -- traditionally the kickoff of the big summer driving season.

"People are conserving, but it's important to not take for granted the supply," said Chuck Clinton, the District of Columbia's top energy official. "We're still not swimming in gasoline."

A major question that has some oil analysts puzzled is whether the reawakening of competition in gasoline retailing is going to force refiners and retailers to absorb some of Carter's 10-cents-per-gallon "conservation fee."

If they do, the public will not have to suffer the full impact of the fee. At the same time, the fee would not have the full effect that Carter desires on forcing Americans to use even less gasoline.

Emond, of the Lundberg Letter, thinks the fee will hit dramatically at the pumps about May 15 because the oil companies will pass it through all at once in an effort fo convince the public that the higher costs are Carter's doing and not theirs.

After that public display, the oil companies might allow prices to soften as they fight for market shares in a situation of abundant supplies.

There is widespread opposition across the country to the fee, and an effort in Congress to kill it altogether.Vic Rasheed, head of the Greater Washington-Maryland Service Station Association, called the fee "a ripoff of American motorists who are already conserving to their limits."

Members of Rasheed's organization are asking motorists to sign petitions opposing the fee.

Robert W. Sullivan of the National Oil Jobbers Council, a trade organization, said the fee will add $10.3 billion to the cost of gasoline products during the first year it is in effect. Howard Mencher, legislative assistant for the AFL-CIO, called the fee "an attempt to balance the federal budget on the backs of working men and women."

At a recent lunch at The Washington Post, Exxon chairman Clifton Garvin said that Carter's fee plus possible world crude oil price increases and the decontrol of domestically produced crude oil prices could add 37 cents a gallon to gasoline prices by Christmas.

That would put the price of gasoline in the Washington area at about $1.65 a gallen by then.