The 97-nation trade talks, which have dragged on with little progress for more than three years, have finally begun to develop a "vague sense of momentum," according to Alonzo L. McDonald, the chief U.S. negotiator. McDonald told reporters he thinks the Geneva talks could reach substantial agreement on liberalization of tariff and non-tariff barriers to trade by late next summer.

He said he expected the negotiating nations would agree on tariff cuts on industrial and farm products that average about 40 per cent.

The Swiss have proposed tariff reductions averaging 44 per cent and the U.S. would be willing to accept average tariff cuts ranging from 39 per cent to 49 per cent. McDonald said.

However, the Europeans are likely to hold out for the lower end of that range, he told reporters.

McDonald said the United States is unlikely to come to the trade table again for at least a decade, so whatever agreements are reached in Geneva will shape international trade for the 1980s.

The administration's negotiating mandate expires January 1, 1980, but McDonald said an agreement should be reached long before that. Congress will have to ratify portions of the agreement.

Under a timetable proposed by the President's Special Trade Representative Robert Strauss, countries will make requests on what non-tariff barriers they want reduced in what countries on Nov. 1. By Jan. 15, the negotiating countries respond to each other's requests.

After Jan. 15, the negotiators will be faced with "the massive task of matching" requests for reductions and the offers.

Requests for reductions in all agricultural areas, including tariff and non-tariff barriers, also will be made Nov. 1. The non-agricultural tariffs have been on the table for some time.

On Dec. 15 a tentative code for dealing with such barriers to trade as product standards will be put on the negotiating table, McDonald said.

The United States will put together a list of exceptions to the general rules by Jan. 15, outlining those tariff reductions the nation is not willing to undertake.

McDonald said that it is likely some textile industries will be on the exceptions list, especially those industries that are labor intensive.

Developing countries want "a great deal more than the United States can give," McDonald said. He said the U.S. has proposed a package of reductions in tariffs on $2 billion worth of tropical products, but demands some reductions in return on the part of developing countries.

He noted that some other industrialized nations are willing to offer trade preferences to developing countries for a period of 10 years. He said the U.S. wants a trade relation with the developing countries and does not want to use the tradetalks as a tool of general economic aid.

Any U.S. gestures to developing countries must be matched by some move on their part, McDonald said. Unlike the trade preferences proposed by other industrial countries, however, the U.S. concessions would be permanent.

The negotiation, who holds the formal U.S. title of deputy special trade respresentative, said he did not expect the tariff reductions would take effect until the 1980s and that they would be phased on over eight to 10 years, because many countries cannot absorb tariff reductions any faster than that.