Mayor Marion Barry told the leaders of two suburban counties yesterday he might rescind Mayor Walter Washington's post-election decision to take a bigger share of capacity at the regional Blue Plains sewage treatment plant.

Barry made his conciliatory move after a closed meeting on the regional sewer crisis with Montgomery County Executive Charles Gilchrist and Prince George's County Executive Lawrence Hogan. It was the first time the three officials -- all of whom were elected last November -- have gotten together since taking office.

The issue that brought them together -- where to treat metropolitan Washington's ever-mounting sewage flows -- has bedeviled the area's leaders for a decade. With the city and the suburbs all competing for revenue-producing development, the debate over sewers has frequently escalated into legal war.

In an effort to de-escalate the were-fare -- scheduled to resume next Thursday in U.S. District Court -- Barry sais he and his suburban counterparts were trying to reach an out-of-court settlement.

"We agreed on trying to approach this regionally," Barry said after the closed session. "No, we don't have the specific problems solved... But we have a framework."

While he tried to keep his answers general, Barry, under questioning, made his surprising comments about possibly rescinding Washington's Nov. 17 decision to take an additional 5 million gallons worth of capacity at Blue Plains.

Washington's action angered leaders of the Maryland suburbs and Fairfax County -- both of which use Blue Plains -- and prompted the Washington Suburban Sanitary Commission to seek a court-ordered sewer moratorium on the District when its flow reaches 135 million gallons daily, its allocation under 1970 and 1974 regional agreements. The Dirstrict is about 2 million gallons daily short of that figure now.

At a press conference after yesterday's meeting, Barry said: "We (the District) have made a commitment to consider using less then 5 million gallons daily (the additional amount of capacity seized by the District last November)."

Interviewed later, Barry said: "I'm considering rescinding it (the decision). It doesn't mean we don't need it (the capacity) -- heck no."

In December, Prince George's said it was prepared to offer the District some additional sewer capacity, but needed more time to decide the exact amount. Just how much help Prince George's can give was probably a topic of yesterday's closed meeting. All that the principals would say was that "numbers" were discussed.

The significance of the issue was underlined at another meeting yesterday, at which Rep. Ronald V. Dellums (D-Calif.), the new chairman of the House District Committee, said that sewer and water problems have a high priority on his legislative agenda.

Dellums also assigned high priority to financing completion of the Metro subway system.

Dellums met with members of the Congressional Metropolitan Washington Caucus, whose suburban members voiced concern over his support for legislation that would permit the District to impose an income tax on suburban commuters.

That measure -- similar to one defeated in the committee last year -- is on the back burner, Dellums told the legislators, according to an aide, and will be pushed only if Mayor Barry requests it.

Rep. Herbert E. Harris II (D-Va.), one of those who attended, praised Dellum's approach and said the surburbanites agreed to support a measure to set a formula for a predictable annual federal payment to support the D.C. budget. The level of the present payment varies, and is set each year by Congress.

Harris introduced legislation yesterday to grant $1.7 billion in additional U.S. assistance for the completion of Metro plus subsidies for operations and the retirement of subway construction bonds. He sponsored a similar bill last year on which no congressional action was taken.

Without such legislation, Metro would depend upon getting an unspecified share of the nationwide transit budget.