The decision by the Reagan administration and the Federal Reserve to seek to lower the value of the U.S. dollar on foreign exchange markets could help keep interest rates from rising even if economic growth picks up, analysts said yesterday.

If the economy does not begin to expand more rapidly than it apparently is now -- the Commerce Department last week estimated the rate for this quarter at 2.8 percent -- then rates could come down. In particular, the Fed might cut its 7.5 percent discount rate by half a point, the analysts said.

However, the analysts noted that nothing further has been done about reducing the large federal budget deficit, the financing of which has been a factor in attracting foreign capital to the United States. A lower value for the dollar and lower interest rates could be inconsistent with keeping that needed foreign money flowing, they cautioned.

In other words, market pressures might produce rising interest rates, whatever the intentions of the administration and the Fed.

With the money supply growing far more rapidly than the Fed has targeted, many financial market participants had been expecting the central bank not to resist any upward movement in rates that might follow a pickup in the economy and a resulting increase in credit demand.

But in the wake of the agreement this weekend among the finance ministers and heads of the central banks of the five largest industrial nations to seek a weaker U.S. dollar, the analysts believe the Fed is at least less likely to allow rates to go up and perhaps drag the dollar along, too.

"Oh my, yes. This is a new constraint on the Fed," said Scott Pardee of Discount Corp. of New York, a major government securities dealer. "We don't know what commitments have been made," but higher rates would be inconsistent with the whole package, which has the support of President Reagan, Treasury Secretary James A. Baker III and Fed Chairman Paul A. Volcker, he said.

"The market has in its mind that there might be a cut in the discount rate somewhere along the line," Pardee continued. "I am not sure I agree with that, but money growth is the only reason that interest rates are not 50 to 100 basis points lower now." A basis point is 1/100th of a percentage point.

Economist Henry Kaufman of Salomon Brothers said that he thinks the agreement will cause the Fed to keep its current policy unchanged in the near term, with the federal funds rate -- the interest rate financial institutions pay when they borrow reserves from one another -- staying in the 7 3/4-to-8 percent range it has been in for some time.

Volcker is known to have been in favor of more vigorous intervention in foreign exchange markets than has been the administration, which until early this year opposed virtually all such actions. There has been an understanding between the Fed and the Treasury Department that intervention would not occur unless both felt it should.

Thus, Volcker probably welcomed the new willingness of the administration to accept more intervention. Given the way in which the Fed guards its policymaking prerogatives, it is unlikely that the Fed chairman gave any promises about future interest rate movements, said some close observers of the central bank.

But that is an issue to be joined later, if the economy does pick up speed. Fed officials so far have found few concrete signs of the stronger economic growth they predicted for the second half of this year -- or the even stronger 5 percent growth forecast by the administration.

As a result, the emphasis at the Fed has been on keeping interest rates from rising even in the face of a soaring money supply. Fed officials do, however, worry that sooner or later the rapid money growth could generate more inflation, but so far there is no sign of that.

Volcker and other central bank officials have been concerned that the dollar might take a nose dive and quickly cause prices to jump by making imports more costly and by removing some price competition for U.S.-made goods. In July, Volcker was at some pains to reassure financial and foreign exchange markets that acceptance of the rapid money growth of the first half of this year did not mean the Fed had dropped its concern about inflation. An expectation of accelerating inflation in the United States likely would drive the dollar much lower, Fed officials said.

But the Reagan administration and the Fed both fear that a continued high value of the dollar could lead to passage of new measures to protect American manufacturers directly from foreign competition -- measures that themselves could add to inflation and perhaps invite retaliation by other countries.

The agreement struck last weekend suggests that Volcker has found that prospect more of a concern than the possibility of a sharp dollar decline.

As for the direct impact of a lower dollar on the demand for U.S.-made goods, it could be a while in coming. "A reasonable rule of thumb to use in this connection would be around nine to 12 months," said Salomon Brothers' Kaufman. "At that time, it would have a positive influence on domestic economic activity, improve some profit margins and add at least marginally to the inflation rate."