Plans to build a conference center and corporate headquarters for the Howard Hughes Medical Institute on one of the last large parcels of land in Chevy Chase has divided neighbors in this expensive community off Connecticut Avenue and raised the specter of intense development pressure in the area.

Some, like Guido Adelfio, president of the Chevy Chase Hills Citizens Association, say the two-story conference center and office building is preferable to the 68 single-family homes allowed under the current R-90 zoning.

"The Hughes project is a far-sighted benefit to the community," Adelfio said. "It's a low-density usage that will keep the rolling character of the site and retain the huge trees."

But Maryanne Berberich, president of the Chevy Chase Valley Association on the north side of the parcel, doesn't see it that way.

"I can't believe very many people feel the way Guido does. There's a lot more at stake than just the trees," Berberich said. "This is the first attempt to develop that land in 20 years."

The last serious attempt was thwarted when citizens sued and stopped developers who wanted to locate a Bloomingdale's department store on one of the twin 20-acre parcels straddling Connecticut Avenue at Jones Bridge Road.

Now residents say rumors are circulating that Chevy Chase Land Co., which owned the tract purchased by the Hughes Institute, has development plans for two other parcels it owns in the area.

A spokesman for the land company said the two parcels, 20 acres opposite the Hughes tract and another eight acres that was the location of a water-powered electrical plant used to fuel the trolley system until the 1920s, have not been sold.

"There is intense interest in the parcels, but then there has been since 1890," the spokesman said.

This renewed interest is what worries residents up and down the congested Connecticut Avenue corridor. "The whole Chevy Chase Lake area is about to become a focal point for development in this part of the county," said Jane Lawton, vice chairman of the Town of Chevy Chase council and president of the Bethesda-Chevy Chase Advisory Board.

"We are very upset about development by special exception, and that is what this is," Berberich said. She said her group supports current efforts by the planning board to change county zoning law so that medical facilities like the Hughes Institute can't build in a residential neighborhood on parcels larger than two acres.

Berberich says the proposal by the Hughes Institute will aggravate traffic on roads already rated as poor or inadequate by county transportation engineers. She said residents also fear construction by Hughes in the neighborhood will help justify county plans for a transit way with light rail or bus service on the old Georgetown branch of the B&O railroad right-of-way, an idea many adamantly oppose, she said.

Robert A. Potter, director of communications for the Hughes Medical Institute said his group is "very much aware" of citizens' concerns about traffic and is working with the county to see what measures they can take to mitigate any adverse impact. The rolling, wooded site appeals to the institute because of its "retreat-like campus setting. We would hope to preserve that feeling by building low-rise facilities," he said.

The Hughes Institute was formed in 1953 at the same time as Hughes Aircraft, which the reclusive billionaire industrialist gave to the medical institute. In 1985, Hughes Aircraft was sold to General Motors Corp., giving the Medical Institute a current endowment of $5 billion, Potter said. The institute is required to spend 3.5 percent of the endowment annually "directly on research" in order to keep a favorable tax status, said Potter.

In 1987, the Hughes Institute moved its headquarters from Coconut Grove, Fla., and is leasing space in Bethesda. "We want a permanent site where we can bring together the 178 scientists we employ at various medical institutes and universities across the country," he said. The executive offices and conference center eventually will house about 200 employes, he said.

Potter said evidence of the institute's commitment to build structures that preserve and enhance the surrounding community can be seen in its recent renovation of the historic 1922 Sisters of the Visitation convent, known as the Cloisters, on the National Institutes of Health campus.

Jane Lawton said no development should be allowed to go forward on any of the three vacant parcels between Manor and Jones Bridge roads on Connecticut Avenue until the Maryland Highway Administration conducts a major traffic study of existing roads. "We don't want to get into the same mess as Wisconsin Avenue," which is heavily developed in Montgomery County, she said.

The county is approving a lot of development, but since East-West Highway and Connecticut Avenue are state roads, it is often hard to get traffic studies in time to alleviate congestion, according to Lauton.

Guido Adelfio said after meeting with Hughes Institute representatives that he is confident "they're not going to build a big, splashy place with neon lights. It's kind of a reclusive organization. And their plan would tear up the tract a lot less than 68 homes."