France and West Germany, attempting to downplay strains in their once warm relations, today pledged joint measures to fight the "extremely high level" of U.S. interest rates that are contributing to Western Europe's economic woes.

French President Francois Mitterrand and West German Chancellor Helmut Schmidt disclosed no details about the measures agreed to at the 39th semiannual summit meeting between leaders of the two countries.

Both countries have blamed their economic distress on the Reagan administration's monetary policies and have warned that high interest rates in the long run could prove more dangerous for the future of the Western alliance than the Communist crackdown in Poland.

While the two leaders put on a public display of friendship in denouncing interest rates, one American diplomat said practical action was limited by basic differences in French and West German outlook and by the fact that the two agreed to consult with fellow members of the 10-nation European Community before taking any steps.

"They may have something up their sleeves," said an American diplomat after the meeting, "but it must be pretty high up."

Considered more important than any single point in the joint communique was the general impression both sides sought to convey of renewed cooperation between Western Europe's two most powerful countries.

In the months immediately following Socialist Mitterrand's sweeping election victories last spring, Paris had shown signs of distancing itself from West Germany, in part because of the extremely close relations that Schmidt had maintained throughout the 1970s with Mitterrand's opponent, conservative former president Valery Giscard d'Estaing.

One symptom of a new French mood toward the Germans was an announcement that the Mitterrand government had backed off plans to nationalize Roussel-Uclaf, the French subsidiary of Hoechst, the German pharmaceutical giant.

A compromise was announced to coincide with the summit that eventually will reduce Hoechst's share of the company but still leave the Germans in control. Schmidt's government had backed Hoechst in the dispute, reflecting Bonn's dislike of nationalization.

The two governments today also downplayed differences over Poland by condemning "repressive measures" there and demanding the lifting of martial law.

Mitterrand stressed France and West Germany's commitment to the Atlantic Alliance but said the two countries reserve the right to make independent decisions. That was interpreted in some diplomatic quarters as serving notice that the two countries would not be deterred by American criticism from going ahead with the controversial Soviet gas pipeline project.