In the three decades since Xerox Corp. began developing a wide swath of riverfront land east of Leesburg, Loudoun County has been transformed from rural fringe to booming suburban center.

Once-lazy Route 7, which runs by the property, is a major thoroughfare dotted with office parks and apartment buildings serving a population that has jumped more than 400 percent since the early 1970s.

Now, a controversial effort by local officials to slow that growth -- not only with zoning restrictions but also by banning development on environmentally sensitive land -- has drawn the Connecticut-based document giant and a host of other high-profile firms into a legal battle with the county.

Xerox is one of more than a dozen Fortune 500 companies, national home builders and major planned communities that joined local landowners and other opponents last week in filing more than 150 lawsuits to block all or part of a far-reaching new slow-growth law that the Board of Supervisors passed last month.

"We are saying Xerox properties should be exempted," said spokesman Bill McKee. "We do expect to be successful."

Critics have long vowed a court fight to reverse the supervisors' sharp reduction in the number of homes that can be built per acre in the western two-thirds of the county. That "downzoning" was the basis for most of the nearly 200 lawsuits filed against the county so far. But for Xerox and many other major firms, the underlying zoning was not cut. Instead, those firms oppose the new law's stricter environmental rules.

The companies are asking to be exempted from regulations that prohibit construction near waterways or on sensitive natural features such as flood plains. The other companies include Dallas-based Centex Corp., a home-building and financial services company with $8 billion in revenue last year; Toll Brothers Inc., the Pennsylvania-based luxury home builder; Washington banking concern Riggs & Co.; and the developers of many of Loudoun's large commercial and suburban developments, including Lansdowne, Cascades and Brambleton.

Property rights protections are broad in Virginia, and county officials and plaintiffs' attorneys agree that the new regulations cannot cancel previous building approvals and the "vested" right to build that accompany such approvals. What remains to be seen, however, is whether -- or in which cases -- the courts would rule that a new environmental regulation automatically deprives a developer of his rights.

"I believe some folks are vested, and some folks are not. We'll have the opportunity to sort through it," said Scott K. York (R-At Large), the board chairman, adding that the county will not be able to apply the new environmental rules to many properties that are already far along the building approval process.

For example, York said, if the county has given specific approval for the construction of a 50-home development, "we can't come back and impede their ability to put those 50 houses on a site." He added that if a developer could show that the new rules denied it the ability to build what was previously approved, then it would be allowed to operate under the old rules.

Many of the recent lawsuits are unnecessary and are based on misguided fears, York said.

"I think there were some attorneys trying to drum up business, and at the end of the day, the panic button that went off wouldn't have needed to go off," he said.

Some lawyers said that they had reluctantly advised their clients to file suit to protect their rights and that they hope the county will offer written assurances for specific properties that would allow them to drop the suits.

"The problem is they couldn't get me a letter by Feb. 5," said lawyer Deborah C. Welsh, citing a 30-day deadline for filing many kinds of suits that she wanted her client, developer River Creek LLC, to meet. "I'm hoping I get a letter that takes care of all this."

Welsh said that although the county did not change the number of homes that River Creek could build per acre, the new environmental rules could limit their size.

Supervisor James G. Burton (I-Mercer) argued that the new rules actually benefit developers because, unlike the old rules, they provide building credits to compensate for avoiding sensitive land. Those credits can be used elsewhere on a property, he said.

"Most of these companies that are complaining already wind up in a better situation than before," Burton said. "We tried to explain it to them during the process, but most of them, their eyes glazed over and they didn't understand."

Over the years, Xerox's real estate subsidiary has sold off all but about 40 of the nearly 2,300 acres the company once owned in Loudoun for a series of developments, including the upscale Lansdowne on the Potomac.

Hobie Mitchell heads the Lansdowne development and filed suit against the county.

"I have never sued a government entity before. I've been in this business for 30 years," Mitchell said.

He hopes to be able to drop the suit once he is given a written guarantee that the development, which he said already voluntarily met strict environmental standards, is not subject to the new rules. "It's billions of dollars' worth of assets everybody's trying to protect," Mitchell said.