Plaintiffs led by the Teamsters union, consumer group Public Citizen and the Environmental Law Foundation sought an immediate injunction to prevent the trucking rules from taking effect on May 3.

The expanded Mexican trucking, approved by Congress last year under provisions of the North American Free Trade Agreement between the United States, Canada and Mexico, is scheduled to begin as early as this summer.

The U.S. Transportation Department is currently assessing Mexican truck inspection plans, safety monitoring programs and border state law enforcement. No Mexican trucks can operate beyond a narrow commercial zone near the border, where they currently do business, until the analysis is completed in late June.

The groups are challenging the Bush administration's finding that Mexican diesel truck emissions will have no significant impact on public health. They want the government to conduct a more detailed environmental review.

"By failing to accurately assess the true environmental impact of allowing these trucks from Mexico throughout the United States, the administration has put the health of millions of Americans at greater risk," Joan Claybrook, president of Public Citizen, said in a statement.