Rockwell International Corp., the giant defense and electronics firm, has reached a tentative agreement to buy 68 acres in western Fairfax County for a major office complex that could employ as many as 3,200 workers, according to county officials.

Rockwell, the 29th-largest corporation in America, with 1989 sales of $12.6 billion, has a contract to purchase land at Westfields Corporate Center, a huge office park south of Dulles International Airport, for up to 1.5 million square feet of office space, according to correspondence released yesterday.

Before finalizing the purchase, however, the company has asked for and received assurances from Fairfax County that the amount of development allowed would not be reduced by the county's controversial downzoning law, which was overturned last week in court.

A Rockwell spokesman said that the purchase contract allows for a study period, in which the company can make sure the site is suitable, and that a decision will be made in a month or two. The spokesman said the company hopes to use the space for electronics work.

Coming amid a sustained slowdown in the regional economy and complaints from business leaders that Fairfax is "anti-business," county officials said, a decision by Rockwell to locate such a large operation in Fairfax would be a major coup. It could be the largest corporate boost to the county since Mobil Corp. decided to move its world headquarters here in April 1987.

"It's wonderful for our economy, and it's an opportunity for employment and prestige," said Supervisor Elaine N. McConnell (R-Springfield). "This company is a gem that will help boost our economy when we're at a low ebb."

Supervisor Thomas M. Davis III (R-Mason) said it was "good news for our taxpayers and our economy at a time when we're hearing how sluggish it is."

The move would give Rockwell, the nation's 12th-largest defense contractor, a major corporate foothold at the doorstep of the federal government. The company, builder of the B-1 bomber, also works on some of the military's most sophisticated satellite systems, produces parts for the space shuttle and the MX intercontinental ballistic missile, and has a hand in numerous top-secret communications projects.

Details of the company's possible Northern Virginia expansion were scant. Rockwell, which has headquarters in El Segundo, Calif., now mploys about 200 people in the Washington area, most in offices in Crystal City, according to company officials.

"Washington is one of our major market sources. We do a lot of business with the government, and we want to be close to them," said James McDivitt, senior vice president for government operations.

McDivitt said Rockwell is considering Westfields as the home for a new electronics business and is not considering a major relocation of its other divisions to the site, which he said would be developed in phases.

He added that if the company backs out of the purchase, "we're not committed to buying any property in the Washington area. If this falls through, then I don't know what we do." He said it would be "a few years at least" before operations at the site could begin.

A source familiar with Rockwell's plans said that at least part of the company's desire to expand in the Washington area apparently was driven by contracts that Collins Defense Communications, a Rockwell subsidiary with offices in Cedar Rapids, Iowa, and Richardson, Tex., obtained with the CIA. Peter Earnest, a spokesman for the CIA, declined to comment.

William H. Keech, senior vice president for Henry A. Long Co., which is developing Westfields, declined to comment.

Rockwell's search for a home in the area has been underway for months but became public only yesterday, when Fairfax County released a letter assuring the company it would be allowed to develop the Westfields site to its full potential.

The letter was made public after the Board of Supervisors was briefed on its contents during a closed session yesterday afternoon.

Concern apparently had been raised by the company that last December's downzoning by the County Board of Supervisors, or litigation prompted by the development restrictions, could prohibit the company from developing the land with 1.5 million square feet of space.

The downzoning, which reduced development rights on 14,000 acres of commercial and industrial land, was overturned last week by a Circuit Court judge who ruled that it was vague, improperly drafted and riddled with mistakes. The board discussed the ruling yesterday but delayed a decision on whether to appeal it until the judge enters a written order in the case.

In a letter sent to Rockwell officials yesterday, County Executive J. Hamilton Lambert said that Westfields had been exempted from the downzoning because of public improvements that the developer of the office park had promised to provide.

In addition, the letter noted, because Westfields is in the Route 28 Special Tax District, it also is protected by state legislation passed this year exempting projects in the district from downzonings.

Lambert added that if other complications arose, he would support expedited applications to allow for the full development of the site, adding, "It is my expectation that such applications would be approved by the board."

"Fairfax County wants you to locate here and to construct your project on Parcel 41" at Westfields, the letter states. "We will work with you and continue to be helpful in solving any issues that may arise during the site plan approval, construction, occupancy and use phases of your project."

The 68-acre sale would be one of the biggest commercial real estate transactions in Northern Virginia in months and would be a boon to Fairfax, which has a 16.9 percent office vacancy rate.

Other corporations at Westfields include Ford Motor Co., Martin Marietta, Contel Federal Systems and American Systems Corp., a defense contractor. Rockwell's operation would be the biggest there.

Keech said about 700 of the 1,100 acres at Westfields have been sold.

Staff writer George Lardner contributed to this report.