The Marriott Corp. is negotiating to build a major motel-convention center at Tysons Corner, an area that has grown so rapidly that it is being called "the downtown" of Fairfax County, principals in the proposal have acknowledged.

The motel, which would have 450 to 500 rooms and "considerable public space," would be built next to the planned Tysons II Mall which, when it is finished in about 1982, will make Tysons Corner the largest shopping complex in the Washington area and one of the largest in the United States.

"There are complications," said a source familiar with the Marriott negotiations, "but it's likely to go through."

The northwest sector of Tysons Corner, 1,700-acre area bounded by the Capital Beltway, Rtes. 7 and 123 and the Dulles Airport Access Road, is in the midst of rapid development.

The site that interests Marriott is part of a 107-acre Tysons 11 tract on Rte. 123, across the road from the present shopping mall. It would be easily reached by ramps that will be part of a freeway-style interchange that will carry traffic to and from both Tysons I and II.

John Bacanskas, Marriott's vice president for development, said he would "neither confirm nor deny" that the company is planning to build self Lee's strongest opponent in the Democratic primary, charged at a press conference yesterday that the anti-Venetoulis commercials signaled a "mood of desperation" in the Lee camp. "Blair Lee's activities this week," he said, "prove that when the going gets tough, he (Lee) instincwhat would be its first motel in Fairfax County.

At present, Marriott has a total of 1,375 rooms in metropolitan Washington, making it the second biggest hotel operation in the area after the Holiday Inn chain. Marriott has announced plans to increase its total of rooms in the Washington area to about 3,000 by 1982.

If built, the Tysons Marriott would be the largest hotel in Fairfax County. A nearby Holiday Inn has 243 rooms and will be expanded by about 100 rooms within the next four years. A Best Western motel with 350 rooms is being built in the same area.

The largest hotel in the county is the 302-room Sheraton Inn in Reston.

H. Max Ammerman, who with Theodore H. Lerner is the codeveloper of the Tysons II project, said, "We've made no deal with Marriott. We've discussed it with them . . . I never count on anything until I see it in black and white."

After years of delays the Fairfax County Board of Supervisors in June rezoned the 107-acre Lerner-Ammerman tract to permit construction of shopping center and a motel.

The major complication in the Marriott negotiations, according to a source, is where the highway interchange on Rte. 123 will be built and when it will be finished.

Ammerman and Lerner have agreed to provide the land and $3 million for construction, but the Virginia Department of Highways and Transportation has yet to present a timetable indicating when the project could be started.

Without the interchange - which would ease traffic bottlenecks on Rte. 123 - neither the shopping center nor the motel could go forward.

"Tysons Corner is the primary business center of Fairfax County today," David A. Edwards, executive director of the county's economic development authority, said recently. "In fact, you can truthfully call it our downtown."

The source familiar with the Mariott negotiations put it more broadly: "Tysons Corner has become the headquarters of Northern Virginia."

By 1980, office space there is expected to grow from 3.8 million square feet to 6.2 million square feet. The construction of Tysons II Mall will double shopping space to about 3 million square feet.

At present Tysons Corner is choked with traffic - 50,000 cars pass through the area daily and Tysons II is expected to generate 30,000 more - but major transportation improvements are expected by 1983, and there is even a chance that Metro rail will be extended to the area if the full, 100-mile line is built.