Once upon a time, nearly everyone living in Northern Bethesda was glad to have the National Institutes of Health and the National Naval Medical Center for neighbors. It was small wonder.

The campuses of the two institutions grace the area with sloping lawns and pleasant architecture. They also employ nearly 20,000 relatively well-paid people, who on weekdays turn lunch-hour shoppers and restaurant patrons along downtown Bethesda's commercial strip.

Meanwhile, whenever a president goes in for a checkup, and often when a Nobel Prize is awarded, the two medical institutions bring front-page renown to Bethesda. And the fact: that so many white-collar NIH and NNMC employees live in the adjacent Maplewood and Alta Vista subdivisions has greatly helped to shore up the resale value of the $100,000-plus houses there.

But peaceful coexistence in Maplewood and Alta Vista has been on the wane since April.

That was when the U.S. Department of Health and Human Services announced it was studying the feasibility of building a $100 million coal-fired electricity and heat generating plant to serve NIH and NNMC.

The plant would produce heat and electricy at the same time, at relatively little cost, by a process known as "cogeneration."

The Bethesda cogeneration plant, the only one of its kind under consideration for a major federal installation, would be the second such plant in the Washington area. Georgetown University operates a similar coal-fired plant, but its output is only 1/25th that of the plant being considered for NIH and NNMC.

The feasibility study, which is costing $294,000, was undertaken because President Carter ordered all federal agencies to reduce fuel use by at least 20 percent by 1985. In addition, the oil-burning furnaces at the two Bethesda installations are more than 30 years old and need to be replaced.

The Bethesda cogeneration plant could not open before 1985, and 1987 is "probably more like it," according to Max Novinsky, manager of special energy projects for HHS and chief planning coordinator of the Bethesda plant. Nor is it certain where the plant would be built, or even on which campus it would be built.

But the two major changes a coal-fired power plant could bring to the quiet, treelined streets of Maplewood and Alta Vista -- uncreased air pollution and a sharp increase in traffic -- have already upset many of the area's 2,5000 residents.

"Absolutely incrediable," said Marie McGrath, who live about eight blocks from the main gate of NH. "If I wanted coal trucks in my neighborhood, I'd go live in West Virginia."

"I can't believe they'd do this in the middle of such a populated area," said a young woman pushing a baby carriage through the Maplewood-Alta Vista Recreation Center one recent afternoon. "Is saving some energy worth our health?"

"We know that the (new) plant would reduce the country's dependence on oil, so we have to admit that what we're fighting is the wave of the future," says Lester Hubbell, a retired Navy admiral who is president of the Maplewood Citizens Association.

But we don't want to see heavy industry in a residential neighborhood. And there isn't any question this would be heavy industry."

Although planning has not gone beyond the study stage and Novinsky stresses that no firm decisions have been made, this much about the plant seems likely:

It would consume approximately 200 tons of coal daily, if coal is the chosen fuel, and if there is no significant increase in the amount of power needed. If the coal is brought to Bethesda by truck, during daylight but not during rush hours, a vehicle would have to arrive or depart every 4 1/2 minutes, every day of the year -- to deliver coal or to take away waste products.

If coal were used, it would be stored in three silos, at a still-undetermined site on one of the two campuses. Each silo would be approximately 100 feet high and 65 feet wide. A hundred feet is the equivalent of about 10 stories -- above the tree line on many parts of the two campuses and thus possibly, within a sight of some residents of Maplewood and Alta Vista.

If trucking fuel to the site proves politically or economically difficult, fuel might be shipped by a B&O Railroad freight line to Chevy Chase. From there, it could be ground into coarse particles the size of table salt, mixed with water and then shipped via an underground "slurry" line to NUH and NNMC. However, the most direct route for the slurry line would take it under several homes in Chevy Chase and under the back nine of the Columbia Country Club golf course, making further local protests likely.

While Novinsky insists the air pollution from the cogeneration plant "could be kept to a minimum," Montgomery County director of environment planning David Sobers says he plans to "look over their shoulder."

The reason: Maplewood and Alta Vista already have the highest average levels of hydrocarbons of any neighborhood in the county, thanks largely to auto traffic on Rockville Pike, Old Georgetown Road and the nearby Capital Beltway.

According to Hubbell and other neighborhood residents who have monitored the cogeneration plans closely, truck traffic is the paramount concern.

"We just got out from under all the heavy trucks from the Metro construction," said Jackie Friedewald, who lives three blocks from the Naval Medical Center. "They really are a major interruption. They really are loud. Trucks as a steady diet would really by intrusive."

"I wouldn't feel safe letting my children cross the street if there really were going to be that many trucks," said an employe of the Swiss Embassy who lives in the neighborhood and asked that her name be withheld.

"We've got a lot of retired people in the community who deserve a little peace and quiet," said a retired Marine Corps general who lives on Chandler Street.

"The long and the short of it is that we've got traffic problems right now without this kind of thing," said Hubbell. Indeed, more than 40,000 vehicles use Rockville Pike between the Beltway and Cedar Lane each weekday -- one of the highest such figures in Montgomery County.

While no one has talked of lying down in front of bulldozers ("This community isn't like that," Hubbell said), Maplewood residents have protested to Rep. Michael Barnes (D.-Md), and at least a few are talking about legal action to block the cogeneration plant.

To Novinsky, such reactions are "regretable but understandable."

Even the plant's sharpest detractors concede that Novinsky and his consultants have been open about the project. "If I were the federal government," Hubbell conceded, "I wouldn't even bother with some community group."

Although the federal government is technically immune from Maryland and Montgomery County laws at its 500-acre Bethesda complex, Novinsky says, "We will be in compliance with every environmental law. I think we can take care of emissions and effluent without any difficulty."

While a nuclear power plant at Bethesda would neatly remove the traffic issue, Novinsky said he "can't imagine it in view of the feeling of people in general. We just can't consider it." Nor would solar power be economically feasible for such a large facility, according to several experts in the field.

One option HHS is considering for the Bethesda plant is to subcontract its construction and operation to a large, private company -- probably the Potomac Electric Power Company.

Vince Cushing, Pepco's manager of corporate planning, said Pepco is "interested in cogeneration anywhere" -- not because it could make a profit from an operation like the one in Bethesda ("We couldn't," Cushing said), but because any electricity generated on the Bethesda site does not have to be provided by a plant elsewhere.

"To keep rates down, we are always looking for ways not to have to build more capacity if possible," Cushing said, "and this would be a good one."

County official Sobers said he is "not in a position to say we endorse [the plant] at this time. The whole question comes down to how well they can do it.

"The county isn't viewing cogeneration as a bad idea. But we wouldn't want to be where the mistakes are made, either."