Acting President Jose Sarney, charged with replacing gravely ill president-elect Tancredo Neves, says that he is determined to pursue Neves' economic austerity policies and that the renegotiation of Brazil's huge foreign debt will decide the fate of the country's first civilian government in two decades.

Despite Neves' worsening health and the resultant political uncertainty in Brasilia, Sarney said there would be no change in the new government's determination to begin its term with potentially painful measures against inflation, which has reached an annual rate of more than 230 percent.

Sarney also emphasized that "there is a limit to the sacrifices that can be asked" from Brazil's 130 million people. He said creditors must accept the new government's need to address urgent social problems and the threat they pose to its political survival.

Sarney, who was sworn in as vice president 11 days ago, made the remarks in an interview here yesterday with The Washington Post.

His comments took on particular significance today as Neves was rushed to surgery for the third time since the eve of the government's March 15 inauguration. The 75-year-old leader was transported from Brasilia to a hospital in Sao Paulo early this morning after doctors discovered hemorrhaging in his small intestine.

After tests at the Sao Paulo Heart Institute hospital, Neves underwent surgery for more than four hours this afternoon as doctors sought to halt internal bleeding. An official bulletin said the hemorrhage and an increase of blood pressure caused the rupture of the scar from the first of two previous operations carried out on Neves' intestine.

Late today, presidential spokesman Antonio Britto announced that the operation went "very well" and that "the president is fine." Tonight, however, government sources voiced concern over Neves' ability to recover from the stress of a third abdominal operation.

Neves, elected with Sarney last Jan. 15 by a special electoral college, frequently has been described by political leaders here as a deft master of politics whose personal leadership is essential to maintaining both the policy and the overall cohesion of the broad governing alliance.

However, interviews yesterday and today with Sarney, Foreign Minister Olavo Setubal and Central Bank President Carlos Lemgruber indicated a substantial consensus among government leaders to carry out Neves' austere and relatively conservative plans for the economy even in his absence.

Under the constitution, Sarney, 54, may continue as acting president indefinitely while Neves -- who has not been inaugurated as president -- is incapacitated. In the event of Neves' death or permanent disability, Sarney would be formally sworn in as president.

The new administration plans to begin negotiations in early April with the International Monetary Fund, which suspended its economic program with Brazil last month because of a failure to meet key economic targets, including those for the inflation rate.

Brazil must reach agreement on an economic plan with the IMF in order to reopen negotiations with banks on the rescheduling of about $50 billion of its $100 billion in foreign loans. Officials hope to complete talks with banks by May 31, when an interim agreement will lapse.

Last week, the new administration announced measures to control inflation, including a 10 percent cut in the budgets of government agencies, a freeze on hiring and a suspension of credit by state banks. Central Bank President Lemgruber described the actions as "a very signficant signal of austerity" that would be well received by IMF authorities.

Sarney, however, stressed that Brazil has decided to adopt the new belt-tightening polices "as our own decision" and "not as an imposition from abroad." While agreeing with IMF officials on the need for tough action against inflation, he said the government also believed that "we have a social debt that is much larger than the foreign debt."

"We cannot accept a demand for a commitment that we cannot meet," Sarney said during the interview at the modern Planalto Palace with Katharine Graham, chairman of the board of The Washington Post Co., and a group of editors and reporters.

"The foreign debt has created impassable barriers," Sarney said. "There are limits of health, of hunger, of education. They are the limits of survival. If we do not take these into consideration, our plan will collapse."

"The success or failure of our government will depend on our ability to renegotiate the debt," he concluded. "After 20 years of military rule, we are mindful that we cannot fail."

Lemgruber said that in the short term, Brazil will continue to negotiate with the IMF and banks "within the framework" established for managing the debts of Latin American nations since 1982. The process has involved formal agreements with the IMF on measures to increase export earnings, reduce imports and combat inflation while loan payments are rescheduled over longer periods.

Lemgruber indicated that the new government may vary little from the past military administration in the repayments terms it seeks for its loans. At the same time, he said, the administration will seek "more realistic" economic goals with the IMF and restore the credibility of a country that has failed seven times in two years to carry out its IMF plans.

This pragmatic short-term strategy will be blended with more aggressive action by Brazil to reform the international financial system in conjunction with other Latin American debtors, Foreign Minister Setubal indicated. He said "international action" to change the terms of debt payment "will have to take place to find a solution."

Setubal said that the debt problem "will not be resolved simply by the increase of exports or by normal market developments," as the Reagan administration and other western governments have argued. Instead, he said, political negotiations would be necessary to create broad new conditions for the payment of debt by all Latin nations.