Trend of the economy

''If you thought that inflation was coming down quickly,'' said Charles L. Schultze, ''the third-quarter figures would have disabused you. If you thought that inflation was declining slowly and stubbornly, those figures were confirmation.''

Thus former President Carter's chief economic adviser, now a senior fellow at the Brookings Institution, interprets the latest government reports on price performance in the United States.

Measured broadly by the gross national product deflator, says the government, inflation rose 9.4 percent in the third quarter of 1981, compared with 6.4 percent during the April-June period.

Measured more narrowly by the consumer price index (CPI), inflation jumped 13 .5 percent at an annual rate during the third quarter, compared with 7.4 percent in the second.

Even after discounting for flukes and aberrations, these reports indicate strongly that the core rate of inflation in the US economy still hovers not far below 10 percent.

This fact has major implications for the shape in which the economy finally will emerge from the current recession, especially where interest rates are concerned.

Normally, during a recession both inflation and interest rates decline, reflecting reduced public demand for goods and credit. As consumer spending progressively dries up during an economic downturn, lenders lower their rates to attract borrowers. (This, of course, has not always been the case; sometimes inflation especially stays high during an economic slump, causing what some economists call ''stagflation.'')

However, some economists do expect both rates to decline during the current recession, which most analysts expect will persist at least into the spring of 1982.

For two reasons, however, interest rates - which now constitute a powerful drag on the economy - may not dip as far as people hope:

* Lenders, anticipating huge government borrowing into the future and high future inflation, may keep rates up to protect their returns on loans.

This fiscal year, for example, President Reagan - far from reducing the budget deficit - may chalk up the largest shortfall in US history, larger than the $65.6 billion recorded in Gerald Ford's last presidential year.

Deficits on this scale mean that the US Treasury will be shouldering its way into capital markets for a long time to come.

* New proof that inflation is still alive and kicking may stiffen the resolve of Federal Reserve Board chairman Paul A. Volcker and his fellow Fed governors to keep credit tight.

Mr. Volcker insists that too-early relaxation on the monetary reins would send out a message that Washington simply is not serious about controlling inflation.

Strict controls on the growth of the nation's money supply - the essence of current Fed policy - puts upward pressure on interest rates.

To put all this in perspective, inflation and interest rates should come down somewhat during the recession, but - judging from the somber inflation figures of the third quarter - the decline may not be as marked as originally had been hoped.

Dr. Schultze sees a ray of hope in hourly earnings figures, which indicate that wage increases are averaging about 8 percent, considerably lower than in recent years.

Prolonged recession almost certainly would dampen wage expectations of union leaders in the major bargaining year coming up in 1982. Already, for example, General Motors and Ford want the United Automobile Workers (UAW) to reopen their contract and accept wage and benefit concessions, to aid their distressed employers.

Such concessions are becoming fairly widespread throughout the economy, analysts say, as workers face a choice between lower wages or no job at all.

Although the recession is bad news for the White House on the budget front, it may have the effect of postponing a potential crisis for Reaganomics, some critics believe.

Mr. Reagan's tax cut program was designed by White House supply-side economists to whip up the economy, producing both jobs and fresh tax revenues.

Such briskness, however, would have required vast amounts of capital to finance, colliding with the Fed's determination to keep the money supply tight. This combination - when coupled with enormous Treasury borrowings to finance the debt - foretold a competition for capital that might drive up both inflation and interest rates.

Recession now postpones that possible scenario, because the economy is shrinking, not growing. Down the road, however, the test of supply-siders vs. critics may come, when the economy - fueled by next July's 10 percent tax cut and huge defense spending - takes off again.