WASHINGTON -- The head of the Environmental Protection Agency yesterday repeatedly refused to say how soon he will comply with a Supreme Court ruling and decide whether to regulate carbon dioxide, the leading gas linked to global warming.

EPA Administrator Stephen Johnson, appearing before the Senate Environment and Public Works Committee, was asked to provide a timetable for responding to the April 2 Supreme Court decisions. The court said the Clean Air Act makes clear that the agency must regulate carbon dioxide if it is found that it endangers public health.

The legal argument has been settled, and "now there is an unmistakable green light to take action now," Sen. Barbara Boxer, D-Calif., the committee's chairman, told Mr. Johnson.

But Mr. Johnson called the court's ruling complex and said he did not want to be tied to a specific timetable.

When Ms. Boxer said EPA staff had indicated that the agency could make a decision on regulating carbon dioxide emissions from motor vehicles in three or four months, Mr. Johnson said he would "not commit to a specific four-month schedule."

While the court's decision focused on tailpipe emissions from cars and trucks, Mr. Johnson said the agency is evaluating what impact the ruling might have on the need to regulate releases from power plants and industrial sources as well.

Mr. Johnson sought to deflect some of the criticism by telling senators that the EPA earlier yesterday had taken another step toward considering whether to grant a waiver to California, so that it could implement regulations to reduce carbon dioxide emissions from tailpipes.