Rejecting a strategic arms limitation treaty would amount to "an open invitation" for the United States and Soviet Union to hide their military activities from each other, Defense Seretary Harold Brown said yesterday.

Speaking at Carleton College in Northfield, Minn., Brown added that argument to the ones the Carter administration has been making in its campaign to convince the public that signing the SALT II pack now being negotiated would make the world safer.

The strategic arms control agreements the two superpowers have signed in the past, Brown said, have made it easier for the United States to keep track of what strategic weapons the Soviets are developing and deploying.

In both the antiballistic missile and SALT I pacts of 1972, the United States and Soviet Union agreed "not to interfere with the national technical means of verification"-including the spy satellites both sides have deployed-and "not to use deliberate concealment measures which impede verification by national technical means. . . ."

Those provisions, Brown said, would be preserved in the SALT II draft which has been negotiated. But "if these provisions were to expire" through Senate rejection of the treaty, Brown said, "it would be an open invitation to countermeasures, camouflage and cheating of all sorts."

One possible countermeasure which has provoked concern in both Washington and Moscow is the deployment of weapons to kncok down the picture-taking satellites each superpower flies over the other's territory. Separate U.S. Soviet negotiations are under way in hopes of outlawing antisatellite weapons.

Through satellite photography and other monitoring of Soviet military activities, Brown said, the United States has been able to verify whether the Soviets are living up to the arms control agreements.

"The verifiability issue," continued the defenses secretary, "comes down to a single question: Can we detect violations before their impact would hurt our security? I am confident that we can and will continue to be able to do so."

Without a SALT II, Brown said, the Soviets "would almost certainly deploy more strategic systems - perhaps as many as one-third more."

The United States, he continued, would have to respond in kind, resulting in "a renewed, more costly and potentially destabilizing strategic arms competition. And verification of what the Soviets were doing would be no less necessary - merely more difficult."

Brown has said repeatedly even after approval of a SALT II, the United States will have to spend more on strategic weapons in the future than it is spending today. But the increase would be even bigger without the treaty, he has asserted.

"Without SALT II, we would almost certainly enter an era of greater military and political uncertainty that would result in increased strategic forces on both sides," Brown said in the text of his Northfield speech, which the Pentagon released yesterday.

SALT II "will mean greater stability and predictability in the strategic challenges we face and, as a result, will enable the balance to be maintained at a substantially lower level of destructive power than would otherwise be the wise be the case," Brown said. Hawkish opponents of SALT II contend it would freeze the United States into an inferior military position, while dovish critics complain that the new weaponry the United States will buy to make the treaty acceptable is too high a price to pay.