The ruling answered a last-minute request by a coalition of environmental, labor and trucking firms for an emergency stay on President Bush's lifting the moratorium on Mexican trucks in November 2002.

Judge Kim McLane Wardlaw wrote that "although we agree with the importance of the United States' compliance with its treaty obligations with its southern neighbor . . ." that compliance shouldn't "come at the cost of violating United States [environmental] law."

The coalition argued that the U.S. Department of Transportation hadn't made a thorough enough study of the environmental impacts Mexican trucks might have on the U.S., particularly border towns, news agencies reported.

U.S. and Mexican environmental rules are essentially the same, but U.S. environmental regulations will get stiffer in 2004 and 2007 and Mexican rules will not, according to the Associated Press.

An attorney for the International Brotherhood of Teamsters, one of the coalition groups, said Mexican trucks are generally older than American ones and more polluting, AP reported.

But Mexican carriers counter that the older models are mainly the drayage vehicles that only travel the restricted commercial zones in the U.S. and then return to Mexico.