Gasoline lines virtually disappeared at midmorning in the Washington area yesterday for the first time in seven weeks.

No one knows exactly why, but observes said the odd-even rationing plan may have helped. In addition, several major oil companies have slightly increased July supplies of gasoline to service stations.

"The lines this morning were the shortest in two months," said Arlington Sunoco dealer Lewis Haskell. Only two days earlier, on Saturday, there had been long lines and figths over gasoline at his station.

At a Merit station at 6th Street and Florida Avenue NE, 19 cars were in line at the 7 a.m. opening. By 9 a.m., there was no line.

There was a line of 3 to 5 cars at Greenbriar Arco just south of Chantilly in Fairfax County yesterday. In recent weeks lines there had numbered as many as 35 cars.

Lines have also virtually disappeared in cities elsewhere.

Federal officials and oil company executives have long said that psychological factors played an important part in the formation of lines - and by the same token in their virtual disappearence.

Once drivers go through the initial trauma of waiting in lines and of uncertainty about getting gasoline, and once public officials respond by imposing odd-even rationing and other plans, people tend to settle down and adjust to driving less.

Then the lines go away.

That's the theory that federal energy officials have put forward, anyway. They say that's the pattern California drivers followed after long gas lines first formed there in late April.

By early May, lines had appeared in the Washington area and soon after began cropping up in metropolitan areas all over the country.

The complex federal rules governing distribution to cities' problems by tending to drain gas away from them into th small towns and rural areas. Often the extra gasoline was not needed there.

But while psychological factors may be work, the actual quantities of gasoline major oil actual companies are supplying to stations have increased this month over last.

One of the Washington area's largest gasoline retailers, Sunoco, yesterday announced a 5 percent increase in July supplies to its service stations.

Amoco, Exxon and Mobil also slightly increased July gasoline supplies. Together, the four companies supply about 850 stations here - about half the Washington metropolitan area stations.

They are increasing supplies this month by a range of 2 to 5 percent.

Of the other big gasoline retailers here, Gulf and Texaco are supplying about the same amount as last month. Only Shell has announced a slight decrease for July.

The companies gave several reasons for the increases.

Sunoco and Amoco are buying gasoline on the world spot market at high prices that will be passed on to motorists at the pumps.

A Sunoco executive said the company is following this policy because its customers want more gasoline, even if they must pay soaring prices for it.

An Exxon spokewman said its July allocation increase resulted from a slight decline in demand by priority gasoline customers - farmers and others who are allowed by federal rules to get special allotments off the top of the nation's gasoline supplies before the rest is divided among motorists.

Exxon spokesman speculated that the end of the planting season may have caused a decrease iin farmers' demands for gasoline.

Other company spokesmen said some refineries are running at higher capacity, that imports of crude oil by their companies have slightly increased or that they are using up stocks of crude oil to make more gasoline.

Some Washington motorists were confused by the lack of lines yesterday.

"Are you open?" asked one surprised motorist at the Sunoco station at 2205 Pennsylvania Ave. SE.

"Yes," attendant Bob Allen said.

"I didn't see any lines, I though you were closed," the motorist said, happily pulling up for a fill.