Exxon and Texaco said yesterday they plan to begin exploratory seismic testing for oil in the ground under the Potomac River and the Chesapeake Bay, drawing immediate complaints from environmental groups concerned about the health of the troubled estuary.

Ron Jarvis, a spokesman for Exxon, said the testing will be conducted in January and February on a 30-mile stretch of the Potomac about 40 miles downriver from Washington and in a 30-mile area of the bay, from the mouth of the Patuxent River north almost to the Bay Bridge.

Jarvis said the joint effort, following up work done a year ago by an independent seismic testing firm, is still "very preliminary" and would not affect commercial fishing or recreation on the water.

But Will Baker, executive director of the Chesapeake Bay Foundation, said his organization opposes commercial exploration as well as drilling or refining activities on the bay, the country's largest estuary. "One problem we don't have is major oil spills," Baker said. "To introduce the potential for a whole new contamination seems the height of idiocy."

Jarvis said, "The companies are very aware of and sensitive to environmental concerns on the bay." He said seismic testing involves sending a radio wave from a boat into the rock strata below. The sound wave bounces back and paints an electronic picture.

"Preliminary knowledge gained from analysis of rock samples," he said, "suggests a relatively thick rock strata that could hold oil or gas." He said the seismic testing requires no special permits. The bay and river bottom are owned by Virginia and Maryland and any drilling would require state approvals.

The present concern about the bay grew out of a $27 million federal study in 1983 that suggested a relationship between water pollution and the loss of underwater grasses, which nurture and support aquatic life. With oysters and rockfish in sharp decline, officials fear that failure to deal with Chesapeake pollution could further affect the bay's important fisheries.

The oil company plans were disclosed by Baker as state and federal officials were preparing to conclude an interstate bay clean-up agreement that environmental groups already have criticized as grossly inadequate.

"The {oil companies'} timing seems particularly bad now when the governors have reaffirmed their commitment to saving the Chesapeake Bay and have demonstrated how difficult it is to reduce pollutants in the bay," Baker said.

His foundation was one of 16 environmental groups that held simultaneous news conferences yesterday in Richmond, Annapolis, Washington, Harrisburg and Norfolk to criticize the draft interstate agreement on the bay as toothless.

The bay agreement is slated to be signed Dec. 15 in Baltimore and a meeting of state officials to discuss the document's final form is scheduled for today.

"This agreement will seal the fate of the Chesapeake Bay cleanup for years to come," said Charles Fox of the Environmental Policy Institute. "In far too many important areas, it produces plans and not action. We hope it will be strengthened."

In Annapolis, John Kabler of the Clean Water Action Project said the agreement "just needs to be more precise. It needs to have more deadlines and specific numbers."

The draft plan, hammered out in Norfolk in August and subjected to extensive public review this fall, was widely hailed at the time as a a major step by Maryland, Pennsylvania, Virginia, the District and the federal government in attacking the seemingly intractable problems of bay pollution.

The draft's most stringent section called for a 40 percent reduction by the year 2000 of nutrients, such as nitrogren and phosphorus, entering the bay from sewage plants and farmland runoff. Excessive amounts of these elements can kill fish and shellfish over time.

The environmentalists embraced the 40 percent reduction, but found the plan lacking in specifics aimed at curbing the amounts of toxic chemicals dumped in the bay, protecting wetlands and reducing and recycling waste.

David Carroll, an aide to Maryland Gov. William Donald Schaefer on bay matters, told the Associated Press that the final draft will not set specific targets for industrial discharges of toxic waste.

"They want us to move into what we consider a pretty dangerous area," he said. "Number one, you would get sued right away" by industries that would challenge the right of states to set limits.