Mario Vazquez Rana -- whose bid for United Press International was accepted this week -- is no stranger to adversity, as far as bankrupt news organizations are concerned.

When the Mexico City businessman purchased the Mexican Publishing Organization nine years, the chain of 36 newspapers was $78 million in debt and was, in his words, "a bigger problem than UPI is now."

Vazquez changed all that, paying off the debt, pumping millions of his own dollars into the chain, and acquiring another 34 newspaper companies.

Vazquez is today the largest publisher in Mexico and oversees a news empire that he says is worth some $500 million.

Now the 53-year-old magnate is hoping to pull the same trick at UPI, the news agency that has been struggling to escape from bankruptcy since April.

On Tuesday, Vazquez won the battle for the agency with a $40 million bid that won the support of UPI management, the wire service union, and a committee of the company's creditors.

While Vazquez was reportedly the frontrunner in the final days of talks, he clinched the deal after agreeing to take on as a junior partner Joe E. Russo, a Houston financier who had also bid for the agency.

The plan is being opposed by UPI's majority stockholders, but final approval by the judge overseeing the bankruptcy is not expected to be a problem because the main parties involved support it.

Vazquez's acquisition of UPI will be only the latest in a long line of high-profile enterprises that have thrust the businessman into the limelight as one of Mexico's leading public citizens.

Vazquez built his fortune through his family's business, a large retail furniture chain, before branching into newspapers, and the assets he controls are believed to amount to more than $1 billion. His estate in Mexico City reportedly contains a private soccer field, tennis courts, an indoor waterfall and a bear in a cage.

Vazquez, an expert marksman, is also active in the international Olympic movement, serving among other positions as the president of the Association of National Olympic Committees.

With the aid of his Lear jet, Vazquez travels the world in support of his various activities. He numbers among his friends Cuban leader Fidel Castro, whom U.S. Olympic officials say Vazquez lobbied to reconsider his boycott of the 1984 Summer Games.

Before returning to Mexico yesterday after a week of marathon negotiations, Vazquez gave a brief interview in which he pledged as much money as necessary to return UPI to profitability, as well as his intentions to take a hands-off approach to the news agency's editorial functions.

"My wish is to enlarge UPI's prestige, and improve its services by 1,000 percent," Vazquez said, speaking through an interpreter.

Although there has been much speculation that UPI's new owners will try to expand the company's business into new lines related to information technology, Vazquez left no doubt that first priority is to beef up the agency's traditional wire service operations.

He said that UPI's equipment need modernization, and that he plans to open more news bureaus. He also ticked off several areas of UPI's coverage that he felt could be improved, including financial news, sports, entertainment and investigative journalism.

Vazquez stressed, however, that he does not want to get involved in the day-to-day management of news decisions, saying they were best left in the hands of UPI's editors. "I would like to be informed, but I'm not going to interfere," he said. Repeating sentiments he related to UPI's senior management, Vazquez said he wanted "UPI journalism to be clearer, but it will never be manipulated."

One difficulty that Vazquez may encounter as a result of his new expansion is an image problem among some Mexicans, who may resent his poring resources into a U.S. firm when there are many problems to be solved in his own country, some sources said yesterday. Vazquez, who said he has been close to five Mexican presidents, brushed off these concerns.

"If, instead of UPI, I bought a building in the United States, they can criticize me in Mexico because we have a lot of buildings for sale," Vazquez said.

"But UPI is the one I wanted to turn around. There's only one UPI for sale, and it's in the United States. I'm a very international man. I love my country. But UPI was selling in the United States. Criticism is from those people who don't understand the expansion of a business."