Gasoline supplies will be plentiful in the Washington area this Memorial Day weekend -- so much so that many dealers are cutting prices and staying open longer hours to sell what they have.

Although motorists here suffered in long gasoline lines a year ago, southeast Washington Exxon dealer James Gouldin summed up the situation this year:

"I'm trying to get rid of it [gasoline]. People won't buy it. We're not selling our allocations. I've even cut the price on self-serve 4 cents a gallon."

The average price of a gallon of gasoline in Washington is now $127.8 cents -- a whopping 42 percent over what it was a year ago and the key reason why demand is down and supplies ample as the heavy spring-summer driving season begins this weekend.

"Each of us seems to be getting by on 9 gallons for every 10 that we used last year," said D.C. energy chief Chuck Clinton. "We're doing well living within a reduced energy budget and we should continue."

Nearly all the 1,500 service stations in the area are expected to be open this Saturday, the American Automobile Association said. The AAA said 59 percent of area stations will be open Sunday and 73 percent on Monday.

Travel associations said gasoline supplies should be readily available on interstate highways during the weekend.

Vic Rasheed, executive director of the Greater Washington Maryland Service Station Association, likened the current gasoline supply situation to "an oasis in the middle of the desert," saying that supplies may not be so plentiful by the end of the summer.

"Anyone who wants to do any heavy driving should do it now," Rasheed said.

However, most industry observers and analysts said gasoline will be plentiful through the entire summer, preventing the kind of sharp price increases that took place late last year and early this year.

A possible exception to that: motorists could see a 10-cent per gallon increase if President Carter succeeds in getting his "conservation fee" on imported oil through Congress and the courts.

There is an effort in Congress to kill the fee, and a federal judge has ruled that the oil fee is illegal. The administration is appealing that decision.

"When they turned down Carter's tax, people felt relieved," said Alexandia Gulf station manager Patrick Doyle. Now, Doyle said, gasoline sales are "picking up. People are driving more."

A Washington Post survey of area gasoline prices this week showed that the average price -- 127.8 cents -- is only three-tenths of a cent above what it was a month earlier and only half a cent above that it was at the beginning of April.

The average price here may soon go up slightly because Exxon, the area's second-largest gasoline retailer with 24 percent of the Washington market, announced a 2-cent per gallon price increase yesterday.

An Exxon spokesman in Houston said that rise was caused by Saudi Arabia's decision to raise crude oil prices $2 a barrel retroactive to April 1. Several other oil-exporting countries are following the Saudi lead.

However, as long as the nation's supplies of gasoline and crude oil remain at near-record levels, the big oil companies and individual retailers may be forced to swallow some world-wide price increases themselves instead of passing them on immediately to consumers.

Ample supply and price competition is forcing refiners to "bank" some costs rather than passing them on, according to a respected petroleum marketing newsletter, the Lundberg Letter.

This means, under federal gasoline price regulations, that if the big oil companies incur costs that they can't pass on right now, they can pass them on later when there is another world-wide shortage and price competition abates or disappears.

Besides longer hours and profit-trimming by some dealers, ample gasoline supplies have led some dealers here to install or reactive self-service pumps.

"I put in self-service when they told me there was plenty of gasoline," said Silver Spring Amoco dealer Ed Culver, who said he would rather "pay" his customers 6 or 7 cents a gallong to pump their own gasoline than spend the same money to hire additional attendants to man full-service pumps.

"We went from a [shortage] up to having all we can sell in two months," Culver said. "If we were short before, where did it come from?"

Analysts say the answer is simply that people are buying much less gasoline at a time when there has been no big interrupton of worldwide supply.

But that could change overnight. Many dealers indicated that their supplies are barely outpacing demand even with today's high prices.