Following are excerpts from the prepared text of President Carter's speech:

. . . This evening I also want to report to you about the highly publicized Soviet brigade in Cuba and about its bearing on the important relationship between our nation and the Soviet Union . . . .

I want to reassure you at the outset that we do not face any immediate, concrete threat that could escalate into war or a major confrontation.

But we do face a challenge. It is a challenge to our wisdom -- a challenge to our ability to act in a firm, decisive way without destroying the basis for cooperation which helps to maintain world peace and control nuclear weapons. It is a challenge to our determination to give a measured and effective response to Soviet competition and to Cuban military activities around the world.

Now let me explain the specific problem of the Soviet brigade and describe the more general problem of Soviet-Cuban military activism in the Third World.

Here is the background on Soviet forces in Cuba: As most of you know, 17 years ago in the era of the Cold War, the Soviet Union suddenly attempted to introduce offensive nuclear missiles and bombers into Cuba. This direct threat to the United States ended with the Soviet agreement to withdraw those nuclear weapons, and a commitment not to introduce offensive weapons into Cuba thereafter.

At the time of that 1962 missile crisis, there were more than 20,000 Soviet military personnel in Cuba. Most of them were also withdrawn, and we monitored their departure. It was believed that those who stayed behind were not combat forces but were there to advise and train Cubans and to perform intelligence functions.

Just recently American intelligence obtained peruasive evidence that some of these Soviet forces had been organized into a combat unit. When attention was then focused on a careful review of past intelligence data, it was possible for our experts to conclude that this unit had existed for several years, probably since the mid-1970s and possibly even longer.

This unit appears to be a brigade of two to three thousand men. It is armed uith about 40 tanks and other modern military equipment. It has been organized as a combat unit, and its training exercises have been those of a combat unit.

This is not a large force, not an assault force. It presents no direct threat to us. It has no airborne or seaborne capability. In contrast to the 1962 crisis, no nuclear threat to the United States is involved.

Nevertheless this Soviet brigade in Cuba is a serious matter. It contributes to tension in the Caribbean and Central American region. The delivery of modern arms to Cuba and the presence of Soviet naval forces in Cuban waters have strengthened the Soviet-Cuban military relationship. They have added to the fears of some countries that they may come under Soviet or Cuban pressure . . .

Over the past three weeks we have discussed this issue at great length with top Soviet officials.

We have made it clear that the presence of a Soviet combat unit in Cuba is a matter of serious concern to us. The Soviet Union does not admit that the unit in question is a combat unit. However, the Soviets have made certain statements to us with respect to our concern:

That the unit in question is a training center, that it does nothing more than training, and can do nothing more;

That they will not change its function or status as a training center. We understand this to mean that they do not intend to enlarge the unit or give it additional capabilities;

That the Soviet personnel in Cuba are not and will not be a threat to the United States or to any other nation;

That they reaffirm the 1962 understanding and the mutually agreed confirmation of this understanding in 1970, and will abide by it in the future. We, for our part, reconfirm this understanding.

These assurances have been given to me from the highest levels of the Soviet government.

Although we have persuasive evidence that the unit is a combat brigade, the Soviet statements about the future non-combat status of the unit are significant.

However, we shall not rest on these Soviet statements alone.

First, we will monitor the status of the Soviet forces by increased surveillance of Cuba.

Second, we will assure that no Soviet unit in Cuba can be used as a combat force to threaten the security of the United States or any other nation in this hemisphere . . .

Third, I am establishing a permanent, full-time Caribbean Joint Task Force Headquarters at Key West., Fla. I will assign to this headquarters, forces from all the military services responsible for exanded planning and for conducting exercises. This headquarters unit will employ designated forces for action if required.

Fourth, we will expand military maneuvers in the region, and we will conduct these regularly from now on. In accordance with existing treaty rights, the United States will, of course, keep our forces in Guantanamo.

Fifth, we will increase our economic assistance to alleviate the unmet economic and human needs in the Caribbean region and further to ensure the ability of troubled peoples to resist social turmoil and possible communist domination . . . .

We are enhancing our intelligence capability in order to monitor Soviet and Cuban military activities -- both in Cuba and throughout the world. We will increase our efforts to guard against damage to our crucial intelligence sources and methods of collection, without impairing civil and constitutional rights.

These steps reflect my determination to preserve peace, to strengthen our alliances, and to defend the interests of the United States . . . .

I have concluded that the brigade issue is certainly no reason for a return to the Cold War. A confrontation might be emotionally satisfying for a few days or weeks for some people, but it would be destructive to the national interest and the security of the United States . . . .

My fellow Americans, the greatest danger to American security tonight is certainly not the two or three thousand Soviet troops in Cuba. The greatest danger to all the nations of the world -- including the United States and the Soviet Union -- is the breakdown of a common effort to preserve the peace, and the ultimate threat of a nuclear war.

I renew my call to the Senate of the United States to ratify the SALT II treaty . . .

Of course we have disagreements with the Soviets. Of course we have conflicts with them. If we did not have those disagreements and conflicts, we would not need a treaty to reduce the possibility of nuclear war between us.

If SALT II is rejected, these disagreements and conflicts could take on a new and ominous dimension. Against the background of an uncontrolled nuclear arms race, every confrontation or dispute could carry the seeds of a nuclear confrontation.

In addition, SALT II is crucial to American leadership and to the further strengthening of the western alliance. Obviously a secure Europe is vital to our own security . . .

I know that for members of Congress this is a troubling and difficult issue in a troubling and difficult time. But the Senate has a tradition of being the greatest deliberative body in the world, and the whole world is watching the Senate today. I am confident that all senators will perform their high responsibilities as the national interest requires. Politics and nuclear arsenals do not mix.

We must not play politics with the security of the United States. We must not play politics with the survival of the human race. We must not play politics with SALT II. It is much too important for that -- too vital to our country, to our allies, and to the cause of peace.

The purpose of the SALT II treaty and the purpose of my actions in dealing with Soviet and Cuban military relationships are exactly the same -- to keep our nation secure and to maintain a world at peace . . .