THERE HAS NEVER BEEN any doubt that Federal Trade Commission Chairman Michael Pertschuk doesn't like television advertising aimed at children and that he believes children need more protection from ads -- particularly from those hawking highly sugared products. So it was not surprising when companies making those products claimed the FTC's investigation into their advertising would be unfair if he presided over it. Nor was it surprising when a judge ruled a year ago that Mr. Pertschuk could not participate in the investigation because he had made up his mind before the evidence was in.

But the U.S. Court of Appeals here has now put Mr. Pertschuk back into the investigation. It did so on grounds that it had not been established clearly and convincingly that he had an "unalterably closed mind" on matters crucial to the FTC's ultimate decision. This standard is a new one for judging when a federal regulator should be disqualified from a rulemaking proceeding. As this application of it indicates, it is a standard almost impossible to meet. If Mr. Pertschuk cannot be challenged successfully in the "kid-vid" investigation, how can any regulator be disqualified? For that reason, the court's decision is likely to be appealed and the Supreme Court may well decide to review it.

In the meantime, the Court of Appeals has helped put the whole regulatory process in sharper focus. Regulators, it said, wear two hats. When they are deciding cases -- determining whether a company or individual has broken a law or rule -- they are like judges, and the principles of impartiality that apply to judges generally apply to them. But when they are making rules -- the goal of the FTC's kidvid investigation -- they are rather like legislators, taking into account must more than the evidence produced in hearings. Who ever heard of an "impartial" legislator? Put in that context, neither the court's new standard for disqualification nor Mr. Pertschuk's participation in this investigation is as bad as it may appear.

It has been a problem of the regulatory agencies all along to keep their adjudicatory and rule-making functions separate and to avoid being turned into courts, with the formalities and technicalities that would involve. The new decision should help in that regard. It may be the only useful result of the kidvid investigation, a proceeding we have long thought the FTC should never have initiated.

It follows, too, that the appropriate check on how the FTC exercises the power delegated to it by Congress is not some standard of impartial decision-making but congressional review of its performance. That is underlined by the fact that Congress is already conducting -- with a bit more vengeance than necessary -- just such a review.