The age of Assistant Secretary of State Langhorne A. (Tony) Motley was incorrectly reported yesterday. He is 47.

When Langhorne A. (Tony) Motley leaves office next month as assistant secretary of state for inter-American affairs, he will probably open a consulting firm with several Brazilian businesses among his first clients.

While on a recent official visit to Brazil, Motley had preliminary conversations with executives of the alcohol fuel industry about advising them here, and also outlined plans for his firm to other Brazilian executives with interests in Washington, according to State Department and industry sources. Motley has dealt with many of these same people as assistant secretary of state.

No arrangements were concluded, and State Department deputy counsel Michael Kozak said Motley "conducted himself in a completely appropriate manner" by notifying Kozak's office of all his contacts and removing himself from considering any diplomatic matters of interest to his prospective clients.

The way Motley has forged his future provides insight into the legendary Washington revolving door that carries many former federal officials onto the payrolls of companies they dealt with officially while in government.

Motley, 57, announced his resignation April 30 from his high-profile post as the Reagan administration's chief salesman for Central American policy. Motley said he wanted to pursue a higher income in the private sector. He agreed to stay on until his successor-designate, assistant secretary for human rights Elliott Abrams, can be confirmed.

Kozak said that Motley came to see him shortly after announcing his departure in order to go over the rules for job-seeking while in office. "He said he wanted to be totally straight and pure on this," Kozak said. "I wish all people here were as careful as he has been."

Kozak said he told Motley the rules: he could talk with anybody about a job, but it should be on his own time, and he could have no official dealings thereafter with anyone he spoke to or the organizations they represented.

Motley, a colorful Alaska real estate dealer known in diplomatic circles for his scrappy and aggressively informal style, is anything but a conventional diplomat. He was born in Brazil, is fluent in Portuguese and was President Reagan's highly popular ambassador there from 1981 to 1983, when he was named to his present post. He still owns property in Brazil and his mother, a Brazilian, lives in Rio.

In his current position Motley is responsible for overseeing the diplomatic aspects of U.S. relations, including trade and tariff issues, with all Western Hemisphere nations south of the Rio Grande. Like many other nations, Brazil is pressing the United States for better trade terms for its exports, and is pushing to expand its alcohol fuel sales in particular.

It is also fighting a preliminary ruling by the International Trade Commission that its alcohol fuel is being sold here so cheaply that the U.S. industry has been damaged. If it stands, that finding would cause alcohol tariffs to skyrocket just as the market starts to expand because of recent Environmental Protection Agency decisions reducing the lead content of gasoline. Alcohol is an effective substitute for raising the octane level of gasoline, and big money awaits anyone who facilitates expanded Brazilian access to that market.

In mid-May, during a four-day official visit that included talks with Brazil's new leaders, Motley attended an informal Sao Paulo dinner of the Alcohol Producers Association of Brazil, SOPRAL by its Portuguese initials. Some who attended, when interviewed in Brazil, said Motley made it clear he would welcome their business but no deals were made.

"Motley's very interested in Brazilian alcohol," said Sergio Coutinho Nogueira, SOPRAL vice president. "At the dinner he told us we should look him up when we next went to the States." Nogueira said there were no agreements, "but SOPRAL has the idea of approaching him when he leaves government."

Questioned by a reporter, SOPRAL president Luiz Gonzaga Bertelli said Motley had told the distillers he plans to open an office in Washington specializing in inter-American commerce and will set up a subsidiary in Rio or Brasilia. State Department officials close to Motley confirmed his plans to become a consultant, and said he "had talks with some people" about it in Brazil.

Motley has had frequent contacts with the Brazilian alcohol industry as assistant secretary of state. Nogueira said Motley received SOPRAL's export committee chairman, Lamartine Navarro Jr., several times at the State Department when Navarro was in Washington to appear as a technical expert for Brazil before the International Trade Commission.

Those contacts have ended, Kozak indicated. "Motley has carried out all the guidelines we've given him," the State Department legal adviser said. "He's instructed his people that all those matters are . . . to be turned over to his principal deputy."

"Motley could really help us a lot, and he's already done so in all of our visits to the United States," Bertelli said. "There's a hypothesis he could work for us and we'd become clients of his office, although obviously it wouldn't be exclusive," he said. SOPRAL has already hired the Washington law firm of Wald, Harkrader and Ross.

Motley said in an interview that he was "very close" to deciding on establishing a consulting firm but did not plan to set up any subsidiary. "In Brazil, obviously I was asked what I was going to do . . . so I outlined for them some of the things I was thinking about doing, different things that interest me."

As part of his official visit to Brazil, Motley also met with Brazil's top "Group of 30" export businessmen, who had visited Washington in March to lobby Congress on behalf of Brazilian alcohol, steel and footwear. Motley "told us what could have been done better, what had been successful, and gave orientation for a second visit" in June, a prominent industrialist reported.

The industrialist, who asked not to be named in order not to "appear too pro-American," said Motley had been extremely careful to stress that he would not do any lobbying himself and could not because of ethical restrictions, but could only provide advice.

"Motley made clear to the alcohol people that he could supply orientation but not a direct lobby," the industrialist said. "He'd tell them who to lobby, but he wouldn't procure a lobbyist himself."

Motley said the Group of 30 had asked him to comment on their trip "because I had met with them, and so I commented on their trip." He added, "Nobody was pitching anybody on any business. Generically they were trying to figure out how to do business better and they asked me to comment."

Motley bridles at any suggestion that he may have acted improperly. "I am fully comfortable with the way I conducted myself in this whole matter," he said. "I don't lose a minute's sleep over it."