RIO DE JANEIRO -- The post of finance minister in Brazil traditionally is reserved for one of a select group of learned men with the right political pedigree, a resume distinguished by scores of scholarly publications and, often, a foreign doctoral degree.

Nominees are almost invariably scions of wealthy and powerful families from the politically privileged states of Sao Paulo, Rio de Janeiro or Minas Gerais.

But President Jose Sarney pushed aside a slate of blue blood pretenders this week and filled the job with a candidate from much humbler stock: Mailson Ferreira da Nobrega, a career bureaucrat who came late to economics and toiled his way diligently through the ranks of government service.

The eldest of 10 children, he is the son of a small farmer from Paraiba, a chronically poor, drought-parched state in northeastern Brazil's political wasteland.

One Brazilian paper called Nobrega a "piano carrier." He is known not for his lofty theoretical work, but for crunching numbers in second-string jobs at the Central Bank and the Finance Ministry.

Nobrega moved to Brasilia 25 years ago as a clerk at the Central Bank, and 10 years later received an economics degree at a local university. He gradually moved up the administrative ladder to secretary general of finance, an important but unsung position.

"When the president invited me," Nobrega joked with reporters this week, "I knew this really was a country of opportunities."

Nobrega, sworn into office last Wednesday, was the first to acknowledge the challenges of his job. "I assume this office in the midst of one of the worst crises of our recent history," he said at his inauguration.

The fourth finance minister in just 34 months, he takes over from Luiz Carlos Bresser Pereira, a noted Sao Paulo academician who quit in December after a turbulent eight-month term.

Like his three predecessors, Nobrega inherits an economy that is sounding alarms.

Inflation in December topped 14 percent and reached a record 366 percent for the past 12 months. Industry in Sao Paulo, the country's industrial heart, grew a meager 1 percent in 1987. The Federation of Sao Paulo Industries predicts that there will be 25,000 layoffs in the first quarter of 1988.

Although Brazil resumed interest payments to foreign banks in December, the moratorium on such payments, which lasted through most of last year, has scared off foreign investors and badly bruised relations with creditors.

But already the new minister has won widespread praise. Sitting in as temporary finance minister, he managed in the last 20 days to smooth over the business community's objections to a new tax package and to establish a new national minimum wage, two traditionally explosive items.

In his first public announcement as minister, Nobrega ruled out a rumored price freeze and rejected "heroic measures" to correct the economy -- a promise welcomed by a society buffeted by one sweeping economic reform package after another.

"He spoke in favor of a free market economy, something I have supported for years," said Ozires Silva, president of Petrobras, the powerful state oil company.

Known as a technician with no binding party loyalties, he is also expected to drop the ideological stance that has prevailed in prior debt negotiations. The last two finance ministers toed the line of the majority center-left Brazilian Democratic Movement Party, which insisted on negotiating the foreign debt without interference from the International Monetary Fund.

Nobrega said last week that the absence of a debt accord forced Brazil into signing short-term rollover agreements, costing Brasilia over the last two years an extra $1 billion in commissions and bank fees.

Central Bank President Fernando Milliet left yesterday for a new round of talks with creditors in New York. Milliet is expected to seek new loans to cover $4.5 billion in unpaid interest obligations accumulated since the February 1987 moratorium, and fresh financing to cover interest falling due in 1988.

He may also renew efforts to strike a long-term accord that would include a debt proposal similar to Mexico's recent plan.

Like Mexico, Brazil would like creditor backing to convert old debt into new government bonds. Brazil would give priority to creditors holding these new bonds in exchange for a writedown on part of its old debt.

A similar proposal by Bresser was dismissed last October by U.S. Treasury Secretary James A. Baker III as a "nonstarter." Creditors questioned the worth of bonds issued by a government that, unlike Mexico, had declared a moratorium on debt payments.

Economists here argue that negotiations abroad will ultimately hinge on Nobrega's success in righting the skewed home economy.

Like his predecessors, he has also made the ritual promises of reining in the budget deficit, running now at a dangerous 5 to 6 percent of GNP.

However, the enormous government bureaucracy has so far proved resilient against austerity campaigns.

Former minister Bresser Pereira, who quit his post after Sarney balked on approving his tough fiscal reform, charged that the president was too "preoccupied with pleasing everyone" in politics to tend to the economy.