THE SUPREME COURT has sent a surprisingly strong message to deregulators in the executive branch: winning an election doesn't mean free rein in rewriting regulatory policies established in previous administrations. Rejecting the administration's thin justification for revoking an automobile safety regulation, the justices put procedure ahead of politics-as- usual in much the same way they did last week in holding legislative vetoes unconstitutional.

Ever since the Traffic and Automobile Safety Act of 1966, administrations have wrestled with the problem of passenger restraints. Mandating seat belts was easy, but people don't use them. The focus shifted to passive restraints, which don't depend on the rider to do anything: such restraints include air bags that inflate explosively on impact and automatic seat belts attached to the doors. As a stopgap, Nixon administration officials tried the ignition interlock, which made it impossible to start the car without buckling up. Consumers screamed, and Congress passed amendments prohibiting that particular approach. Anticipating a revised regulation, they also imposed a legislative veto. Just when a regulation was about to go into effect, William T. Coleman, President Ford's transportation secretary, decided that the issue needed more study. A year and an election later, his successor thought otherwise and issued the "final" regulation. A solution at last? An end to costly uncertainty faced by industry? Not a chance.

Enter the Reagan team, which kept the pledge of candidate Reagan and revoked the rule in 1981. Its key argument was that auto makers planned to use a type of automatic seat belt that passengers could disable, so that the ultimate safety benefits from the regulation would be too few. But the agency offered little explanation of why the sensible response was no regulation at all rather than a patched-up one.

The Supreme Court decided that rescinding or amending a regulation requires as much care and thoroughness as issuing a regulation in the first place: the agency must explain why it decided things the way it did, demonstrating that it considered the relevant evidence, options and arguments in a rational way. This is the basic means of ensuring that executive action is consistent with the underlying statute (like the auto safety act) and not "arbitrary and capricious" (like a knee-jerk opinion for or against regulation). The reasoning behind an agency's new decision will have to be pretty solid in order to counter the evidence and arguments that supported the one it is overturning. Since 1966, in fact, there have been approximately 60 starts, stops, twists and turns on this regulation's journey. Some of those policy shifts might well have failed the court's test.

Courts should try to avoid making close judgments about whether an agency's reasons are persuasive. But it does make sense to hold official feet to the fire by pointing to lapses in logic and suspicious efforts to ignore available choices or relevant information. New appointees need the flexibility to be able to change past policies. But in our scheme of government, only the elected lawmakers have unfettered power to change regulations without trying hard to give sensible reasons.