On the surface, a recent ruling throwing out Governor Pataki's 1999 plan to sharply lower smokestack emissions by New York utilities is a setback. But in reality, it is an opportunity for Mr. Pataki to make good on his pledge to expand regulation of other known threats to the environment. He should not let the chance slip by.

The ruling, by state Supreme Court Justice Leslie Stein, rested on a technicality. The Department of Environmental Conservation was under a deadline to impose on all 32 power plants in New York lower levels of two pollutants that cause acid rain -- sulfur dioxide and nitrogen oxide. It missed that deadline by 31 days.

Had they been upheld, the new limits would have been twice as stringent as the federal Clean Air Act. But with the timetable in front of her, the judge had no choice but to side with NRG Energy, a power company, and several utility unions that had gone to court to block the rules.

Technicalities aside, though, Justice Stein handed the Pataki administration a victory even as she ruled against it. NRG had brought suit on the grounds that the state lacks the authority to impose emission limits. But Justice Stein found that argument without merit. The law clearly gives the state such power, she ruled.

That opens the way for the department to renew its efforts to impose the tougher limits. A department spokesman says it is "fully committed to achieving the objectives of the original regulations."

That's all well and good, but Mr. Pataki should do even more. Instead of limiting the scope of any new emissions package to sulfur dioxide and nitrogen oxide, the governor should include carbon dioxide and mercury emissions from power plants as well. That would be in keeping with the challenge Mr. Pataki laid down two years ago, when he vowed that if Washington would not address CO2 and mercury, he was prepared to have the state enact strict new limits on its own.

Regrettably, there is scant evidence that the Bush administration is poised to act on these emissions anytime soon. To the contrary, one of the major flaws of President Bush's Clear Skies initiative, which the White House has touted as the answer to acid rain, is its lax regulation of mercury emissions. That leaves Mr. Pataki to take the lead. With the court affirming the state's authority to impose such restrictions, there's no better time than now.