PRETTY MUCH ON schedule, President Reagan has gotten aroused by news leaks in the national security area and, through his new national security adviser, has pledged to crack down by "all legal means"--a welcome qualification--on officials who do the leaking. At the Pentagon, no less a figure than the deputy secretary of defense, chasing one especially embarrassing leak, has taken a lie detector test. The promise of a broadly open government, followed by the grim pursuit of the inevitable unauthorized disclosures, seems to be a pattern that all administrations fall into after a time.

We of the press have, of course, a vested interest in open government and in opening up government. Nor does this interest fade in the national security area. On the contrary, journalistic enterprise and competitiveness often increase in that area, not simply because it's newsy but because there the errors attributable to excessive secrecy can be egregious. Precisely because so much security business is conducted behind closed doors, the press is obligated-- if it is to perform well its first duty of informing the public--to find out what it can.

The press does not contend that a government has no claim to privacy in deliberating on national security policy and no claim to secrecy in some of its decisions. But as a practical matter, the press as a whole goes after every legitimate story that comes into view, accepting the restraints imposed by journalistic professionalism but not those imposed by official decree. If secrets are to be kept, it must be the government, not the press, that keeps them.

If there is broad, though not full, agreement in American society on which party should keep the secrets, there is not broad agreement on what constitutes a secret, or a legitimate secret. Unsurprisingly, the government's definition is much more inclusive than that of the press. Mr. Reagan is said to be outraged, for instance, that classified matters that have not even been presented to him for decision are disclosed in the press.

He is in a long line of outraged presidents. Making policy in an involuntary goldfish bowl can complicate the life of public officials. It can also give the public the opportunity to enlarge its influence before policy is set in concrete. This is one of those tensions that, in a democratic society, it is best not to expect to resolve. It helps explain why the press can understand why President Reagan has given the leak-seekers a new charge, and why it cannot cheer them along.