NORFOLK, AUG. 6 -- The often fragmented effort to clean up the Chesapeake Bay received its biggest boost in four years today when officials from Maryland, Pennsylvania, Virginia, the District and the federal government agreed to draft a long-term, regional plan to reduce the amount of some wastes flowing into the estuary by 40 percent.

However, minutes after the accord was announced, Maryland Gov. William Donald Schaefer said he still intends to relax parts of his state's critical area law, which restricts development around the bay.

"We're going to modify ours," said Schaefer, referring to regulations creating a shoreline buffer zone in Maryland and limiting development to one dwelling per 20 acres in ecologically fragile stretches of bayfront property.

"Twenty acres is too much in some areas," Schaefer said in an interview at the conference site here.

William C. Baker, president of the private Chesapeake Bay Foundation, which fought for General Assembly passage of the critical area legislation, was quick to criticize Schaefer's plan, saying: "I wouldn't want to see 1-in-20 fooled with. It's such a key component of the whole bay effort."

Ironically, this latest regional bay agreement, reached Wednesday night during a private dinner and in closed-door meetings that stretched into this morning, sets the stage for Virginia Gov. Gerald L. Baliles to introduce his own version of a critical area law -- even in the face of expected opposition from Northern Neck and Eastern Shore lawmakers.

"It's something that needs to be done," Baliles told reporters.

Baliles' support for state-by-state "development guidelines," rather than the vague "advisory" standards contained in an earlier draft of the bay agreement, was one of a number of 11th-hour breakthroughs that allowed the states to reach this first major milestone since President Reagan committed his administration to the cleanup in 1983.

"On development, Virginia made a major, major concession -- concession, that's the wrong word -- a major, major advance," said Schaefer, who forged an alliance of sorts with Baliles to persuade Pennsylvania to go along with the waste-reduction initiative.

That initiative, which was both the centerpiece of the agreement and the major sticking point during the negotiations Wednesday night, would give the states until the year 2000 to reduce the amount of excess nutrients going into the bay by 40 percent. The nutrients, chiefly nitrogen and phosphorous, enter the bay through sewage treatment plant discharges and farmland runoffs. Over time, excessive loads of these natural elements can kill off fish and shellfish stocks.

The 40 percent reduction will place a special burden on Pennsylvania, whose intensive farming areas lie in the Susquehanna River basin, which provides the bay with about half of its fresh water.

"The agreement sets out high goals and tight deadlines," acknowlegded Arthur A. Davis, Pennsylvania's natural resources secretary and the state's chief negotiator here. But, he added, "Everyone feels it's time to get on with it."

Because the 14-page agreement is still a draft -- it will be refined after upcoming hearings and signed in December -- some sections, including the nutrient-reduction initiative, are short on specifics.

Indeed, officials often seemed more comfortable today defining the agreement by what it would not do. For example, Lee M. Thomas, administrator of the U.S. Environmental Protection Agency, said the states may reach the 40 percent reduction target not by controlling nutrients "at a specific plant," but rather by a series of basin-wide programs.

If the states succeed, "it means that marine life will return to those parts of the bay with no or low oxygen," Thomas said. "Forty percent is a very aggressive figure, a very ambitious goal."

The agreement contains no provision for penalties or sanctions if the states fail to achieve the goals; nor does it address the continuing cost of the cleanup, which so far has cost the states and the federal government $226 million.

Virginia state Sen. Joseph V. Gartlan Jr. (D-Fairfax), a longtime champion of the bay cleanup, called the agreement a "blueprint that gets us down to business, a yardstick by which the states' performance will be measured."

"It's not without risk," Gartlan added. "The big issue is going to be the growth and intense development on the shoreline. If we don't hold down growth -- manage it wisely -- we will have a net loss in the bay by the year 2000."

Schaefer, who was assailed during his gubernatorial campaign last year for suggesting precisely what he did today on critical areas, said loosening those regulations was not inconsistent with the new bay agreement. The governor indicated that he wants greater flexibility than provided by the current law.

"In some areas, 20 acres is not enough" to support one dwelling unit, Schaefer said. As for the agreement itself, he added: "I came with a little bit of pessimism . . . . I am now leaving with the highest optimism I ever had."

D.C. Mayor Marion Barry participated in an opening news conference Wednesday afternoon and attended part of the dinner with Baliles, Schaefer and other officials before leaving, according to an aide to Baliles. Barry was accompanied by aides, who stayed for the conference.