In the 1970s, Rozanne L. Ridgway was known as "Tuna Roz," because every time the State Department sent her to Capitol Hill, she arrived toting charts and maps showing how the great fish migrated up and down the hemisphere's western coast.

It was the period of the "tuna wars" between U.S. and South American fishermen over fishing rights, and Ridgway, first as the State Department's Ecuador desk officer and later as ambassador for oceans and fisheries, was in the middle of it.

Now she is in the middle of far more visible issues: arms control, NATO, the Common Market and acid rain. She is assistant secretary of state for European and Canadian affairs, one of the two highest-ranking women in the State Department. But she has no nickname now.

"She's just Roz, or ambassador, or yes'm," one longtime associate said. "She's too professional for anything else."

Ridgway, 50, once had a higher rank -- counselor for the department, in 1980. "I've only had one token job" assigned merely to showcase a woman, she said: "Counselor. I don't recommend it."

A 28-year career foreign service officer, Ridgway is by all accounts anything but a token now. She took over one of State's most demanding, prestigious bureaus last July from Richard R. Burt, an arms-control specialist known for his razor-edge intellect and his tyrannical management style. One of those who have worked for them both, who begged not to be named, said the bureau is "a much happier place" under her.

"There's much more consultation and discussion. People's ideas are sought after much more than they were before," the official said. He and other subordinates agreed that Ridgway's style is direct, firm and clear -- "she leaves no unintended ambiguities," one said -- and just as smart and politically savvy as Burt's was. "The difference from him is that she is able to do all this without being acrimonious," a third subordinate said.

"She states the policy without the passion" that Burt brought to Congress, said Rep. Robert G. Torricelli (D-N.J.), a member of the Foreign Affairs subcommittee on Europe and the Middle East, which calls Ridgway to testify frequently.

"It's refreshing to have an exchange of ideas without a clash of ideology," he said. "She has that important State Department quality of giving the answers she wants to provide no matter what questions are asked." That, Torricelli said, was sometimes frustrating but "the mark of a genuine professional." He said he has "no idea" what her personal politics are.

Ridgway said she dealt with the office transition "by not thinking about it," and that she has good relations with Burt, now ambassador to West Germany. That was confirmed by subordinates, who added that Ridgway, unlike Burt, also has good relations with her counterparts in other departments, notably Richard N. Perle, assistant secretary of defense for international security policy.

Burt's clashes with Perle over arms-control policy were legendary, but Ridgway and Perle cooperated well as two of four members of the U.S. working group that Secretary of State George P. Shultz took to President Reagan's summit meeting with Soviet leader Mikhail Gorbachev in Geneva last November.

Ridgway led the U.S. team in working out terms of the joint U.S.-Soviet communique that ended the summit. They wrangled with the Soviet group until 4 a.m. over space arms, finally agreeing to say that the two sides would "accelerate" those talks.

Ridgway's coworkers said she won everybody's respect at Geneva for her unflappability, persistence and sense of humor, as well as for her understanding of and attention to the enormously delicate details of the myriad subjects in the final communique.

"It was as much a case of stamina as anything," Ridgway said. "We were basically recording agreements that had mostly been worked out over four months of exchanges" before the summit. Half a dozen people traveled constantly all summer to have "thousands of conversations," she said. "It was just sorting, resorting, keep looking for areas where a common approach might be appearing."

Although the summit proved valuable mostly because it let Reagan and Gorbachev get acquainted, it cemented Ridgway in her superiors' confidence, and it was also the scene of one of her most memorable moments on this job, she said.

"The biggest thrill was when the president stepped out on the patio of the residence to greet Gorbachev as his limousine pulled up . . . . There was this sense of enormous pride, that we were on the edge of something very, very important, and that the United States was prepared."

Ridgway also undertook arms-control briefings for the news media for the first time at the summit, usually as the "senior administration official" who is not identified under the rules of the briefing. She got mixed reviews.

"She was awful at first -- didn't seem to know very much and not helpful," said one veteran reporter who was there. "But she got better towards the end."

"If they think I didn't have my feet on the ground, they're absolutely right," she said. "The issues were very broad and the whole world was watching . . . . There's a fear that I might make the kind of mistake that would be costly to the policy."

But she got used to it, and was the briefer of choice for reporters who joined Shultz's March trip to Europe.

The daughter of a blue-collar worker in St. Paul, Minn., Ridgway earned a degree in history from Hamline University there and immediately joined the State Department as an information specialist. It was 1957, and her assignments for the first six or seven years were the kind usually given then to young women: personnel officer answering mail in Manila, visa officer in Palermo. She worked in international relations at Foggy Bottom and as political officer in Norway before becoming "Tuna Roz" in 1970.

After a stint at the bureau of inter-American affairs and service in Nassau, she was asked in 1975 to be special assistant to the vacant office of deputy assistant secretary for oceans and fisheries while officials hunted for a man to fill the job.

"I refused. I surprised myself," she said. She asked that the fishing industry be told she was available, and its spokesman said she was acceptable. She was named to the post. "Obviously they were more prepared for a woman than the department was," she said.

After being ambassador to Finland, she became counselor in 1980 but went without an assignment during the first months of the Reagan administration, under suspicion from the right wing for her association with the Law of the Sea Treaty. She was one of 29 diplomats whose postings were delayed for months by Senate conservatives, but became ambassador to East Germany in October 1982.

When she first arrived and began the customary round of visits, local mayors and factory managers bored her with long lectures on the government's current issue, which at that time was U.S. deployment of missiles in Europe. "She stopped making those trips and let the officials know that if they wanted to see her in the countryside they'd have to stop giving this lecture," recalled another longtime associate. Most diplomats would have shrugged and let the natives adhere to their local customs, "but not Roz," he said.

Ridgway had a Mustang that she later drove around East Germany for official visits, he continued.

Ridgway married Coast Guard Capt. Theodore Deming in 1983, when he was stationed in Ketchikan, Alaska, and she was in East Germany. "I never thought I'd marry," she said. "Who would put up with a wife gallivanting around the world -- 'I'll be home in three weeks' -- but he does." He is now in Washington as Coast Guard intelligence coordinator.

Ridgway said her priorities are set by President Reagan, and that she has never had to defend a policy to which she was fundamentally opposed. "If I was, I'd leave," she said. She looks on her job as one of managing, both people and resources, and said it is a skill that could take her many places after this post.

What would she like that to be? "It's the kiss of death to answer that question," she said.