MADRID, NOV. 5 -- The United States and Spain have reached a showdown in their drawn-out talks over Spanish demands for a substantial reduction of U.S. military forces in Spain.

Despite a last-minute concession from the Reagan administration, Spanish officials said the seventh round of negotiations that opened today is unlikely to produce agreement. As a result, Spain has resolved to give formal notification on schedule next week that it intends to abrogate a 34-year-old accord governing U.S. military bases here, they added.

"A new horizon is opening," a ranking Spanish official said.

The notification means that negotiators have only six months before the treaty expires May 14 and a one-year pullout period begins for the approximately 12,000 U.S. military personnel in Spain. Officials from both sides have gone out of their way to avoid dramatizing the step, but at the same time they appear to be counting on such brinksmanship to break the impasse before May.

According to U.S. officials, the Spanish bases have grown in importance to the North Atlantic Treaty Organization since Moscow and Washington agreed on removal of medium- and shorter-range nuclear missiles from Europe. Aside from increased emphasis on conventional forces, a U.S. withdrawal now would intensify European fears that the United States is slipping into a process that could decouple U.S. and European defenses, they explained.

The U.S.-Spanish negotiations, although they grew from Spain's particular circumstances, also have become a sort of pilot case for a growing series of problems faced by Washington in maintaining U.S. military forces abroad on the basis of agreements dating from another political era.

The U.S.-Spanish treaty, for example, suffers from what one Spanish official called the "original sin" of having been concluded with the dictatorship of Generalissimo Francisco Franco in 1953. For Spain, now a democracy, the negotiations thus mean "making healthy" its relations with the United States as well as redefining the size of specific U.S. military installations, he said.

Agreements broadly similar to Spain's have been called into question recently in Portugal, Greece, the Philippines and Turkey. In the background of the Spanish talks, therefore, is a recognition that what happens here could affect overall U.S. deployments along the southern flank of NATO and, in a broader sense, elsewhere in the world.

The U.S.-Spanish talks, although they concern a dozen installations in all, have boiled down mainly to Spanish demands for removal of the U.S. 401st Tactical Air Wing's 72 F16 warplanes stationed at Torrejon Air Base just east of Madrid, according to officials on both sides.

Spanish officials said the other U.S. installations have not been thrown into question. In addition to Torrejon and a number of smaller posts, the United States has air facilities at Moron and Zaragoza and an important naval base at Rota.

U.S. officials said Washington re- cently proposed reducing the number of F16s stationed at Torrejon as a compromise designed to foster agreement before next week's notification. But Defense Minister Narcis Serra of Spain said Tuesday at a NATO Nuclear Planning Group meeting in Monterey, Calif., that this offer was, "unfortunately, still insufficient."

A high-level Spanish official noted that U.S. Ambassador Reginald Bartholomew, head of the U.S. negotiating team, had said last Febrary that Washington's "maximum effort" would be to shift the F16s and more than 2,000 U.S. support personnel to Moron to get them away from the capital.

The official said the new U.S. offer thus represents progress, but he insisted that only removal of the 72 aircraft from Spain would satisfy Prime Minister Felipe Gonzalez's government. He declined to specify how fast that would have to happen, however, suggesting some room for compromise on a timetable.

"There has been some flexibility on the U.S. side," he added, "but they have not gone as far as they must."

Gonzalez, whose stand in the talks has remained fixed, repeatedly has insisted that his position is a national mandate that he and his Socialist Workers Party cannot ignore or dilute.

Gonzalez was elected in 1982 on a campaign promise to suspend Spanish entry into NATO pending a referendum on the issue. His centrist predecessors, who had made the decision to take Spain into NATO, had just signed an agreement renewing the bases treaty for five years.

When the referendum was held, in March 1986, Gonzalez won endorsement for entry into NATO, but partly on the strength of pledges that Spain would stay out of NATO's integrated command and see to it that the longstanding U.S. military presence here is reduced.

Spanish and U.S. officials emphasized that they want to avoid the looming crunch next May. But Spanish observers pointed out, and U.S. officials said they understood, that Gonzalez's maneuvering room is limited by public opinion and the pledges he made before the referendum.

"The United States wants to reach a deal with Spain, and we realize the need for reduction {in U.S. military presence} because of the political situation and its history," a U.S. official said.

At the same time, Defense Secretary Caspar W. Weinberger has said the F16s at Torrejon are essential to NATO defenses. Since the imminence of a U.S.-Soviet accord on medium- and shorter-range nuclear missiles, other NATO governments also have begun to express concern to Madrid, informed officials said.

The Dutch defense minister, Willem F. van Eekelen, said publicly last week at a Western European Union gathering in The Hague that the planes should remain in Spain.