The Reagan administration, responding to an appeals court order, said yesterday it will reopen for public comment its nine-month-old decision repealing government rules that would have required air bags or automatically closing seat belts in all new cars.

Although Raymond A. Peck, head of the National Highway Traffic Safety Administration, said he continues to believe that his decision to repeal the rule "was in all respects lawful, supported by the record and fully explained," the agency said that it will comply with the court order to obtain more information to justify his decision.

Last month, the U.S. Court of Appeals here ruled that Peck acted illegally in ending the government's 12-year effort to equip cars with air bags or automatically closing seat belts. Finding that the repeal was "unsupported by the evidence in the record" and based on a "hollow conclusion," the court ordered the NHTSA to reconsider its ruling under a tight timetable.

The NHTSA said yesterday that, if the court approves, it plans to seek further public comment on its proposal and hopes to have a new decision--or be able to justify further its earlier one--within 2 1/2 months after the court approved its plan.

Even so, the NHTSA said that its willingness to reopen the record now does not mean that the agency won't appeal the court's decision later.