Lt. Gen. Edward J. Rowny, the Joint Chiefs of Staff representative on the American SALT delegation for the past six years, testified yesterday that the new arms treaty negotiated by his delegation would establish American strategic inferiority and endanger the nation's security.

Rowny, who resigned from the Army last month so he could speak against SALT II, told the Senate Foreign Relations Committee that it should force a wholesale renegotiation of the treaty. He promised the ocmmittee that the Soviet Union would be willing to renegotiate since "they need it [SALT] more than we do."

"We must make it clear we will not accept strategic inferiority," Rowny told the committee. "The treaty, as it now stands, heavily tilts the balance against the United States. This treaty will not enhance our security."

Rown'y opposition to SALT II has been well known for years inside the small community of experts that follows SALT closely. In December 1977, The Washington Post reported from Geneeva an interview he gave that sharply criticized the treaty as it was then emerging.

At that time officials in the Carter administration began to talk privately of replacing him on the SALT delegation, and there were rumors that he might resign.

But Rowny stayed on until last month, and his intimacy with the negotiating process clearly impressed many senators who heard him yesterday. So did the forcefulness of his presentation and the calm, deliberate way in which he answered senators' questions.

Rowny testified that SALT II would give the Soviets huge advantages over the United States in the number of termonuclear warheads on their land-based milliles - by 3 to 1 - and in the destructive power of their weapons.

He also said he doubted any Soviet leader would risk the "untold damage on the Soviet Union" that would follow if the Russians actually used these advantages in an attack. But he expressed fear that Soviet leaders "would be encouraged to toke greater risks in pursuing political goals" by the strategic advantages he said SALT II would give them.

"Because the treaty does not actually put a brake on the momentum of the massive Soviet buildup," Rowny testified, "the United States will for the first time not be able to maintain essential equivalence or nuclear parity" if it is ratified.

Rowny repeatedly told the Foreign Relations Committee that the United States could have secured a much better SALT II pact if American negotiators had adopted tougher bargaining tactics. "We gave concession after concession," he said.

In a spirited cross-examination, Sen. Edmund S. Muskie (D-Maine) challenged Rowny sharply on this point.

How was it possible, Muskie asked, that all the other officials involved in negotiating SALT II failed to perceive this potential weakness in the Soviet position that Rowny claimed to perceive? "If, for 10 years, we've been represented by people in these negotiations who overlooked obvious opportunities for a better bargain," Muskie said, "that's a serious charge." He challenged Rowny to explain the basis for his view.

The heavy-set general, sitting calmly at the witness table, looked squarely at Muskie through thick eyeglasses and told him: "The people who negotiated this thing in Geneva at times had their hands tied." The pursuit of a treaty, he added was so important to officials in Washington that they made concessions to the Soviets rather than holding to firm negotiating positions.

Was this "a gut instinct," Muskie asked, or did Rowny have "solid evidence . . . available to others" that the Soviets would have made additional concessions? Rowny's replies to this and similar questions centered only on U.S. negotiating tactics; he did not cite specific evidence of missed American opportunities.

"Your gut instincts may be great," Muskie told Rowny, "but I think you must have something more than that to operate on."

Sen. George McGovern (D-S.D.) challenged Rowny to explain how the Soviets could use the advantage he perceived them getting under SALT II if - as Rowny testified - a rational Soviet leader would not initiate a nuclear war.

Rowny replied that the confidence a Soviet leader might feel knowing he had strategic advantages might embolden him to escalate a crisis "not knowing where he was going and not knowing where to pull out" of the escalation.

McGovern said he couldn't see how a Soviet leader who knew that American could retaliate with great destructive power nevertheless could feel bold enough "to do anything that might risk a nuclear exchange."

Rowny, who had already testified that he doubted a Soviet leader would ever launch a nuclear attack, told McGovern that Soviet military literature is filled with references to fighting and winning a nuclear war, and that the Soviets believe they have an effective civil defense program to protect their population.

Rowny was also challenged on his confident prediction that the Soviets would willingly reopen negotiations on SALT II if the Senate forced that to happen. Sen. Richard G. Lugar (R-ind.), who was generally friendly to Rowny's testimony, elicited from Rowny the comment that his own study of Russian history and experience negotiating with the Soviets convinced him they would reopen the talks.

"This is going to be a leap of faith that we take if we reject this treaty," Lugar responded.

Rowny testified that in his opinion the Soviets wouldn't exceed the numerical limits on their arsenal in SALT II even if the treaty is rejected. "They have no need to do so," he said, citing the large warhead advantage they could acquire under the SALT II limits. They would more likely devote their resources to qualitative improvements, Rowny said. At the same time, however, he testified that SALT II would allow the Soviets to substantially add to their number of warheads in 1985, when the treaty would run out - another weakness in the document, he said.

Rowny was the second and lesser-known witness to testify against the treaty yesterday, but his comments seemed to overshadow those of the first, Paul H. Nitze, a former Pentagon official and SALT negotiator.

Nitze used many of the same arguments Rowny employed to criticize SALT II, though he refused to come out flatly against the treaty. His testimony became a snappish confrontation with a series of senators, some of whom Nitze interrupted to make his points.

Rowny noted some positive elements in the SALT II by the Carter administration, Nitze said, "with all its fallacies and implausibilities, can only incapacitate our minds and wills for doing the things necessary to redress the strategic balance." Nitze quoted Lincoln. First we must disenthrall ourselves, then we shall save the country."

Nitze, like Rowny, emphasized statistics he said showed the United States would come out of SALT II in 1985 in a substantially weaker position than the Soviet Union. He said that in 1985 the Soviets would have twice America's capacity to destroy "soft" targets, five times America's capacity to attack the enemy's missiles in their protective silos and three times the total explosive power of the U.S. arsenal.

"Strategic parity is slipping away from us and . . . the Soviets can be expected to achieve meaningful strategic superiority, probably by 1982 and most certianly by 1985, unless we take the most urgent steps to reverse current trends," Nitze said.

Nitze, like Rowny, was asked why the Joint Chiefs of Staff - who testified for SALT II Wednesday - disagreed with both of them on so many points. Part of the answer, Nitze said, was that the chiefs "have been misled by the White House" about the meaning of SALT II.

Nitze said the best outcome of the SALT debate would be the emergence of a clear majority in the Senate that would announce its determination not to "concede military superiority to the Soviets," would change SALT II to make it more favorable to the United States, and would face up to the consequences of both positions. CAPTION: Pictures 1 and 2, SALT critics Nitze and Rowny tell senate panel the pact would weaken United States, boost Soviet Union. James K. W. Atherton - The Washington Post