Following are excerpts from Secretary of State Cyrus R. Vance's opening statement at yesterday's hearing before the Senate Foreign Relations Committee on the proposed SALT II agreement:

We proceed today with the second step in a fateful joint responsibility.

The president has completed a negotiation in the process launched by President Nixon with the first strategic arms limitation treaty with the first strategic arms limitation treaty - SALT I - and continued by President Ford at Vladivostok. The Senate is now called to perform its equally important function of advice and consent on the second strategic arms limitation treat - SALT II.

President Carter has taken a further step along the path marked out by his predecessors. I am sure that the Senate will perform its high duties in a totally nonpartisan manner. For the course our country takes, through this ratification process, will have a profound effect on our nation's security, now and for years to come. . .

First and foremost, we must preserve a stable military balance with the Soviet Union. That is the surest guarantee of peace.

Second, we must have the best possible knowledge of the military capabilities and programs of the Soviet Union. We must know the potential threats we face so that we can deal with them effectively. And we cannot rely upon trust to verify that strategic arms control obligations are being fulfilled. We must be able to determine that for ourselves, through our own monitoring capabilities.

Third, we must sustain the process of placing increasingly more effective restraints on the growth of nuclear arsenals.

Fourth and finally, we must take those actions that will strengthen our alliances and enhance our leadership in the world. . .

[The] SALT II treaty will greatly assist us in maintaining a stable balance of nuclear forces. It fully protects a strong American defense.

Our national defense requires nuclear arms that are sufficiently numerous, powerful, and flexible to deter the full range of potential threats. As an essential part of this, our strategic forces must be - and must be seen as - equivalent to those of the Soviet Union.

The SALT II treaty helps us maintain this balance in two fundamental ways:

It will permit, and in fact aid, the necessary modernization of our strategic forces.

And it will slow the momentum of Soviet strategic programs, thus reducing the threats we would otherwise face. . .

A second way that SALT II serves our national security is by improving our ability to monitor and evaluate Soviet strategic forces and programs.

Verification has been a central concern in every aspect of these negotiations. At every stage we put the treaty to this test: Can we have confidence in its verficiation - that is, can we determine for ourselves that the Soviets are complying?

Let me cite some of these significant new steps:

For the first time, there is explicit agreement not to encrypt telemetric information - that is to disguise the electronic signals which are sent from missile tests - when doing so would impede veriication of compliance with the provisions of the treaty. We would quickly know if the Soviets were encrypting relevant information. This would be a violation of the treaty.

We have agreed that we will regularly exchange information on the size and composition of our strategic forces. This is by no means a substitute for our ability to count for ourselves. But the exchanged data will help us confirm that both parties are interpreting their obligations in a like manner.

We have agreed to rules which simplify the job of counting weapon systems limited under the treaty. For example, every missile launcher of the type that has ever been tested with more than one independently aimed warhead will automatically count against the multiple warhead ceiling - even though some may, in fact have only a single warhead. . .

In the days ahead, Secretary [of Defense Harold] Brown and others will provide, in closed session, the detailed classified information that is required for senators to make an informed judgment on verification. I know this issue will be central to your consideration. It has been central to ours. Let me state very clearly that I am convinced we will be able adequately to verify this treaty - that we will be able to detect any Soviet violations before they could affect the strategic balance. . .

Our allies and friends have made clear to us, publicly as well as privately, that they have a vital interest in the ratification of this treaty.

Defeat of the treaty would be a profound blow to our closest friends. Its approval will benefit our most valued alliances. It will signal continued American leadership for peace.

[We] cannot relistically expert to shift the bargain more in our favor now through a process of amendment and reservation.

Even if it were possible to reopen the negotiations, certainly they would be reopened to both sides. This could lead to thereopening of points that are now resolved in a manner favorable to our interests.

As we move ahead, I urge you not to make premature judgments. Let us first carefully consider the treaty as it now stands. Let us see if your questions do not, in fact, have satisfactory answers. And let us all avoid emotional rhetoric - which can only obscure the real issues.

This treaty is complex. It bears on a difficult and complex relationship. Before reaching a final decision, we - the Senate and the Administration together - have an opportunity for a discussion and a debate that will illuminate our common national goals as well as clarify the terms of the treaty itself.

Finally, as we proceed with a debate which will often be technical, let me express the hope that the nature of our subject will be kept cclearly in sight - the terrible power of nuclear weapons.

Together, the arsenals of the United States and the Soviet Union already hold more than 14,000 strategic nuclear warheads and bombs.The smallest of these are several times as powerful as the bomb that destroyed Hiroshima.

If a fraction of those weapons were ever fired, tens of millions of our people and tens of millions of the Soviet people would perish. Nuclear war would be a catastrophe beyond our imagination - for the aggressor as much as the victim.

This, in the end, is what this debate is about - not pieces on a chessboard or chips on a table, but instruments of mass destruction - even as they are instruments of deterrence. . .

Following are excepts of Secretary of Defense Harold Brown's opening statement before the Foreign Relations Committee:

I would not recommend this treaty if it required us to trust the Soviets. Too much is at stake for us to have to rely on their good will or scruples. The SALT II treaty is designed to assure that we do not need to rely on trust. It is verfiable. While I will not be addressing the issue of verification in detail today, verification will be the subject of separate hearings later before this committee and before the Select Committee on Intelligence, where I will speak in detail.

For the present, I will limit myself to stating my conclusion on the subject: The Soviets cannot gain a military advantage by concealed actions that violate the agreement. U.S. intelligence capabilities to detect violations - and U.S. capability to respond to detection of violations by adjusting and incrdeasing our own programs - are such that before any Soviet secret buildup or other violation could approach a scale that would affect the strategic balance, we would detect the effort and be able to respond appropriately to prevent any advantage from accruing from the violation. . .

I have examined this treaty with care and in the light of what I know about the nuclear armaments that exist in the world today and are likely to exist in the future. My judgment is that this treaty will make the people of the United States more secure militarily than we would be without it. For that reason, I recommend that the Senate give its approval. . .

[We] cannot be militarily secure unless our strategic military forces are at least in approximate balance with those of the Soviet Union. The forces of the two countries are in a position of essential equivalence today.

There are two ways to maintain that equivalence. One is for both sides to add to their nuclear arsenals in equivalent or offsetting ways. The other is for both sides to limit their arsenals or to reduce them on a comparable basis. We have the option to follow either course.Either can maintain our security.

The course of limiting arms is preferable, however, for a number of reasons. First, it tends to make the future balance more predictable and stable, and less likely to become one - sided. Second, it provides more certainty to each side about the current program of the other. Third, it is obviously less costly for both sides. Overall, it is less risky for both sides.

Neither the present balance, nor ongoing Soviet programs, nor the state of arms control agreements, are such that we can avoid substantial defense programs needed for our military security.In fact, we need to increase our present level of such programs overall, regardless of this treaty. But SALT II is a clear and valuable, though limited, step toward curtailing the numbers and types of weapons that can be added by either side, and even towards reducing - by some measures - the number of weapons systems that one side (the Soviet Union) already has on hand. . .

By any reasonable standard, the United States has a credible deterrent today and will have one for the foreseeable future. We have - and will continue to have survivable forces capable and industrial potential, even after an all - out surprise attack on the U.S. We also have - and will have increasingly in the coming years - both the forces and the targeting and employment policies that permit us selective use of nuclear force to respond to more limited actions. In particular, we will have forces whose capabilities will make clear that the Soviets could not improve their relative military position by attack. . .

A simply way to measure the utility of SALT II to our strategic objectives of deterrence and essential equivalence is to return to a fact I mentioned before in passing: Without the SALT II agreement, the Soviet Union could have nearly one - third more strategic systems than with the agreement - instead of the 2,250 delivery vehicles of the treaty, they could have 3,000. And there would be corresponding effects on other measures - including overall throw-weight, weapons numbers and the like. Naturally, we do not know what the Soviets would do in the absence of a treaty, but these higher strategic system levels are well within their capability. They are our reasonable intelligence projections. . .

Concern has been expressed that SALT II ratification will encourage complacency and cause the U.S. to fail to do what is necessary to retain adequate deterrence and essential equivalence in strategic forces. In fact the SALT II debate is likely, I trust, to have the opposite effect. An understanding of the strategic situation will promote awareness of what we need to assure our security in strategic nuclear terms - insofar as security is feasible in the era of thermonuclear weapons, ballastic and cruise missiles and strategic bombers.

If SALT is rejected or otherwise fails to come into effect, it is not certain that U.S. actions on strategic forces (as compared to Soviet actions) would produce a more favorable balance than will result under SALT II. The United States could - and I think would - respond to a heightened competition in strategic arms if we had to. The result, however, would be more weapons, higher costs, and probably less security - for both sides. . .

[It] is my considered judgment that the SALT II treaty will serve the defense interests of the United States. It enhances stability. It allows us the flexibility to modernize our strategic forces to maintain deterrence and equivalence without unnecessarily expensive and potentially destabilizing strategic nuclear arms competition.

I do not doubt our economic or technical ability to compete successfully with the Soviets in strategic weapons. I do question whether such an effort is desirable if we can avoid a part of it through strategic arms limitation. And I do not believe that we would find our security increased after such a further round of competition. CAPTION: Picture, Almost surrounded by television cameras, the Senate Foreign Relations Committee opens hearings on the arms limiation treaty. By James K. W. Atherton - The Washington Post