JUST WHEN it seemed that Loudoun County's Republican-majority Board of Supervisors was prepared to abandon any serious limits on growth, two of the GOP members wisely shifted their ground this week. Republicans Jim Clem and Lori Waters teamed up with the board's two independents and lone Democrat to form a bare majority that would prevent madcap construction in the western two-thirds of the county.

The plan endorsed Wednesday night is not a done deal. As reported by The Post's Michael Laris, the supervisors still must work out precise language, hold hearings that could prove explosive and fulfill certain state requirements. Most important, the voices of reason must hold their ground. In addition to building controls, the new proposal contains a special feature: an option to rezone property in exchange for contributions for roads, schools and other costly projects.

Until this week's outburst of reason, the Republican-led board had been bowing and scraping before developers, blessing plans to accelerate Loudoun's already-torrid pace of growth. According to U.S. Census figures released Wednesday, between April 2000 and July 2004, 24,755 homes were built in Loudoun, the third-biggest construction boom in the nation. Loudoun's population doubled in the 1990s, and it has grown faster than any other county in the United States since 2000. Yet the western area remains relatively undeveloped, making it all the more important to take a deep breath and institute reasonable controls there.

The latest move doesn't govern development elsewhere in Loudoun. Last month, the proponents of unlimited growth moved to take over one key gatekeeping agency: Loudoun's Sanitation Authority, which oversees the water and sewer lines essential for additional development. Supervisor Stephen Snow (R), point man for the developers, got himself chosen chairman of the Sanitation Authority's board. That's not good: In most of Virginia, sewer boards are not headed by local elected officials; to insulate these agencies from politics, the appointees tend to be nonpartisan. Yet in Loudoun, members of previous boards of supervisors, including slow-growth advocate Scott K. York (I), have served on the authority board. So much for independent decisions about utility lines.

Last year, a majority of Loudoun supervisors, led by Mr. Snow and his fellow Republicans, voted to allow the extension of utility lines throughout a 23,000-acre stretch between the suburban eastern part of the county and its more rural western parts. "While growth and change are inevitable," Purcellville Town Council member Robert W. Lazaro Jr. said at a hearing this month, "how we grow and change is not. There is no logical reason for Loudoun County to continue to be the recipient of 25 percent of the entire metropolitan region's development."

Over the years the political pendulum in Loudoun has swung between extremes of slow idle and floor-it growth. Now perhaps a new majority for responsible compromise can hold sway and tap the brakes.