THE U.S. COURT of Appeals for the District of Columbia circuit has declared the concept of legislative veto to be unconstitutional. It appears certain that the Supreme Court will now have to grapple with this issue very soon. It presents an opportunity for untangling the rights and responsibilities of the three branches of government on a subject that has engaged legal and constitutional scholars for years.

Congress has enacted more than 200 laws granting the executive branch the authority to make certain decisions, but retaining the right of one or both houses of Congress to veto those decisions. No presidential concurrence is needed. In the case decided Friday, a three-judge panel of the Court of Appeals held that such a provision in the National Gas Policy Act of 1978 violated the separation-of-powers principle of the Constitution. If Congress wants to reverse an executive decision, wrote Judge Malcolm R. Wilkey, it must legislate that change in the manner set out in the Constitution, with votes in both houses and with the concurrence of the president. Just for good measure, the court found that the legislative veto is not only an invasion by the legislative branch into the territory of the executive, but an unconstitutional intrusion on the exercise of judicial powers by the courts as well. It is the function of the courts, Judge Wilkey wrote, to review agency action and to decide whether that action is in accord with the law enacted by Congress.

It is easy to understand why Congress likes the legislative veto. It allows the legislators to enact laws without dealing with the nitty-gritty details, responsibility for which is delegated to the executive. At the same time, the legislature retains for itself the right to second-guess the agencies on every point and to reverse rules and regulations by the simple action of a single house.

And it is easy to understand why special interest groups like the legislative veto. Burdensome, costly or simply unwanted regulation can be reversed if 51 senators can be won over. No need even to bother with the 435 members of the House or with the president.

But it is easiest of all to understand why the court had objections to the legislative veto. The Constitution is very specific about the structure of our system of checks and balances. "The doctrine of the separation of powers," Justice Louis Brandeis wrote more than 50 years ago, "was adopted by the Convention of 1787, not to promote efficiency but to preclude the exercise of arbitrary power. The purpose was, not to avoid friction, but by means of the inevitable friction incident to the distribution of governmental powers among three departments, to save people from autocracy."

In deciding that the legislative veto enabled Congress to expand its role of oversight to one of administration--a responsibility of the executive--the Court of Appeals proposed a simple alternative means of correcting unwanted regulation: "If Congress has given away too much power, it may, by statute, take it back. . . . Congressional unwillingness to use its constitutional powers cannot be deemed a sufficient reason for inventing new ways to act."