After listening to several hours of testimony Monday by Eugene V. Rostow, President Reagan's choice to head the Arms Control and Disarmament Agency, Sen. Alan Cranston (D-Calif.) looked over at the nominee and said: "I get the feeling you do not really consider arms control efforts with the Soviet Union a very practical way to spend our time."

Rostow, who was approved as ACDA director by a 10-to-1 vote in the Senate Foreign Relations Committee yesterday, replied: "I wouldn't be here if I didn't."

In their own minds, both men undoubtedly believed what they were saying. But their conflicting assessments in the brief exchange reflect just how dramatically the Reagan administration's approach -- at least orally -- to nuclear arms control with Moscow differs from what Congress and the public were accustomed to in the Nixon, Ford and Carter administrations.

In those administrations, during which the first and second Strategic Arms Limitation (SALT) accords with the Soviets were negotiated, Congress could usually find doves as well as hawks among officials ranging from the military Joint Chiefs of Staff to the civilian head of the arms control agency.

But in the Reagan administration, every top civilian official -- including the president and his top White House aides, Secretary of State Alexander M. Haig Jr., Defense Secretary Caspar W. Weinberger, ACDA director-designate Rostow and retired Lt. Gen. Edward L. Rowny, the designate for arms control negotiator -- are not only outspoken hardliners but are on record against the kinds of agreements negotiated by the three previous administrations.

"Our 10 years of experience with SALT and SALT II have been painful and unsatisfactory," Rostow testified Monday. "Our first task therefore is to reassess the role of arms limitation agreements in our foreign and defense policy."

The Senate committee isn't used to hearing a stream of hardline anti-Soviet rhetoric from the arms control agency, which, for most of its 20-year history, has been what committee chairman Charles H. Percy (R-Ill.) called an "offset" to those who just wanted to "arm, arm, arm." But that is what it got Monday when Rostow, the 67-year-old Democrat and law professor who once served President Johnson as undersecretary of state, laid out for the record how the Reagan administration looks at these crucial issues.

Rostow said Reagan "wishes to pursue every feasible opportunityfor genuine negotiations with the Soviet Union on nuclear arms." Now that Congress has agreed to the administration's massive defense reguilding program, he told the panel, "it is now possible and desirable for us to resume the search for balanced and verifiable arms control agreements." This, he made clear, would not be the never-ratified SALT II agreement negotiated by Carter, but rather would be a "fresh start."

Rostow said it would take at least another nine months even to figure out what the United States wanted to negotiate. But at the close of his testimony, under pressure from Percy, Rostow pledged to do his best to shorten significantly the time it takes to get new negotiations started.

But Rostow set forth tough conditions for the new talks he said the administration wants. "The Soviet drive for empire is accelerating in momentum and is becoming more and more difficult to contain," he said. Then he unveiled the first specific administration pitch to the allies to join in a revitalization of the 1947 Truman Doctrine of containment of Moscow as a necessary basis for arms control.

"The western nations have more than enough power and potential power to accomplish that goal. What has been lacking is a shared perception of the problem and the political will to deal with it," he said. By pursuing a policy of containment, he added, the West also "should be able to convince the Soviet Union that the imperatives of survival in the nuclear age demand strict and reciprocal respect . . . for world order and international use of force."

He called for Moscow to be forced to obey the United Nations charter on use of force, another suggestion that prompted Cranston who saw little chance of enforcing U.N. peace rules, to challenge Rostow's sincerity (Cranston cast the only "no" vote yesterday).

As for specific U.S. guidelines on future arms talks, Rostow said "our policy will be to accept only agreements that contribute positively to our own security." Too often in the past, he said, adverse changes in the balance of power have been ignored because of "excessive" hopes for the SALT process.

The United States, he stressed, must have as it minimal goal "a clear, credible and unchallengeable secon strike nuclear capability," meaning the ability to retaliate after being attacked, and no negotiation would be allowed to interfere with that.

The administration contends the United States no longer has second strike capability, which is of importance for two reasons, Rostow argued.

The first is that a retaliatory missile force acts as a deterrent to protect the United States and its allies against nuclear attack or the threat of it.

The second is a favorite theme of Rostow's that may ultimately be much more important. Secure nuclear forces "permit us to use military force in defense of our interests with comparative freedom if it should become necessary . . . ." In other words, a rebuilding of nuclear forces will allow use of conventional forces with greater confidence.

Rostow also said he has little confidence, in principle, in the "bargaining chip" style of negotiations. "We should never again defer actions essential to our security in the hope that the Soviet Union will follow suit," he said. "Equally, we should refuse to settle for cosmetic or ambiguous agreements."

Rostow also suggested that, even before formal negotiations with Moscow, the two nations may need to discuss new ways of verifying the dimensions of each superpower's nuclear arsenal. As weapons get more complex, and new cruise and mobile missiles that are hard to spot and count enter the arsenal, techniques other than so-called "national means of verification," which means spy satellites, may be necessary, he said.