October 23, 1986

The General Motors Corporation reported today that it sustained an operating loss of $338.5 million in the third quarter. But income tax credits and the earnings of its financing subsidiary gave the automotive giant a net income of $264 million, or 56 cents a common share. That represents s 48.9 percent drop from last year's third quarter, when the nation's largest auto maker earned $517 million, or $1.53 a common share. Sales rose a slight 1.3 percent, to $22.8 billion, from $22.5 billion in the 1985 quarter.

Dozens of models in vivid multicolored summer dresses streamed down the runway carrying multicolored bouquets while ''Happy Birthday to You'' was sung on the sound track. It was a rousing finale to Yves Saint Laurent's show, on the last day of the spring and summer openings. It was the 20th anniversary of the Rive Gauche boutique, which began the ready-to-wear era in French fashion. Saint Laurent was the first couture designer to make a big impact on ready-to-wear, emphasizing simple sports clothes such as pea jackets, trousers and raincoats. Recently he has paid more attention to his couture collections than ready-to-wear.

President Reagan today signed into law the most thorough revision of the Federal income tax code in more than 40 years, calling the new tax system ''less a reform than a revolution.'' In a splashy ceremony on the South Lawn of the White House, the President said the new law was ''the best antipoverty bill, the best pro-family measure and the best job-creation program ever to come out of the Congress of the United States.'' About two dozen senators and representatives who had been instrumental in enactment of the legislation joined Mr. Reagan on the platform. The occasion was witnessed by more than 1,000 Administration officials, specially invited corporate executives and lobbyists and tourists who happened to be at the White House. Top Domestic Priority The President made overhaul of the tax system with dramatically lower tax rates the top domestic priority of his second term. He pledged today to ''oppose with all my might'' any attempt to raise those rates.

For several recent weeks, the people of Mexico City coped with a scarcity of eggs. Producers, eager to raise prices because of Mexico's potent inflation, held their eggs off the market. They did so to bring pressure on the Government, which controls the price. ''You can swear on it that they are going to raise the price soon, and that the egg men are hiding the eggs so that people get desperate,'' Dolores Villamil Reyes, a 57-year-old beautician, said during the contretemps. The administration of President Miguel de la Madrid will not fall over the price of eggs, but the shortage was yet another symbol of the deterioration of Mexico's economy.

A State Department report made public today says production of marijuana and opium poppy in Mexico has increased dramatically in the last year. The report, the State Department's midyear update of its annual assessment of drug trafficking worldwide, contradicts the Mexican Government's assessment of its drug eradication program. According to the report, marijuana production in Mexico rose by more than 25 percent in the last year while acreage planted in opium poppy, used to make heroin, grew by one-third. Mexico 'the Largest Source' The report says Mexican drug control programs are faltering despite the increasingly cooperative attitude of the Mexican Government. The statistics, it says, ''indicate that Mexico is once again the largest single-country source of heroin and marijuana imported into the United States.''