DETROIT (AP) _ Sales of domestic cars and light trucks rose 10.8 percent in late October against a year ago, automakers reported Wednesday, quelling for the moment fears that last month's stock market slip scared buyers.

''Sales are really not that much affected by the plunge. Consumers really retained cool heads and did not substantially alter their buying plans,'' said Chris Cedergren, industry analyst with J.D. Power & Assoc. in Westlake Village, Calif.

Automakers said their dealers sold 344,190 domestic cars and light trucks in the 10 selling days from Oct. 21-31, up from 310,651 in the same period a year ago.

The late October figures had been awaited as the first concrete indication of consumers' reaction to the wild fluctuations of the stock market last month.

Makers sold 218,803 domestic cars, down a slight 0.3 percent from 219,567 a year ago, and 125,387 domestic trucks, up a healthy 37.7 percent from 91,804 a year ago, the companies reported.

Foreign makers, who report sales only once a month, said their combined car and light truck sales fell 0.4 percent during October, to 339,811 from 341,230 a year ago. Imported car sales rose 0.7 percent, to 274,717 from 272,722, while imported truck sales fell 5 percent, to 65,094 from 68,508 in October 1986.

Sales of many imported, high-priced models have lagged throughout the year but dropped further during the month.

Cedergren said many foreign luxury makers, such as Mercedes, experience slow sales in October because stocks of 1987 models drop as the companies prepare to introduce 1988 models in November.

Mercedes' sales this October, however, came in 24.5 percent below October 1986 because it introduced its new models in October instead of November last year and they sold well immediately, Cedergren said.

Among U.S. makers, industry-leader General Motors Corp. sold 103,777 domestic cars in late October, down 5.8 percent from 110,141 in the year-ago period. GM's sales have lagged behind 1986 levels by about 20 percent throughout 1987, as have Chrysler Corp.'s.

GM sold 46,115 domestic trucks, up 33.1 percent from 34,641 a year ago. Unlike car sales, truck sales for all makers have stayed well ahead of 1986 levels for most of this year.

GM said its Cadillac luxury car dealers sold 28.3 percent more cars in late October than a year ago.

Ford Motor Co., the nation's second-largest maker, sold 59,502 domestic cars, up 12.6 percent from 52,844 a year ago. Ford's truck sales rose 56.7 percent, to 47,637 from 30,398 a year ago.

''At this time, I'm encouraged by the results in October. The underlying economic factors are still the same as before the crash,'' Ford Vice Chairman Harold Poling said Wednesday, pointing to low inflation, falling interest rates and low, stable fuel prices.

No. 3 Chrysler sold 4.2 percent fewer cars during the period - 32,736 compared with 34,160 a year ago - but 18.8 percent more trucks. Chrysler's truck sales, which include Jeep Corp. vehicles, rose to 28,366 from 23,877 a year ago.

Honda's sales of cars made in the United States fell 7.9 percent, to 14,662 from 15,916 a year ago, Honda's first decrease since it began building cars here. Cedergren said the drop was the result of a production halt this year during retooling for new models and made up the difference with imported models.

Volkswagen's domestic car sales were off 14.7 percent from a year ago, to 2,241 from 2,628 a year ago.

Nissan's U.S.-made car sales were up 15.7 percent, to 3,531 from 3,051, as buyers responded to generous incentives on 1987 models. Nissan's truck sales jumped 51.5 percent, to 3,269 from 2,158 a year ago.

Mazda Motors Corp., which began building cars in the United States at its Flat Rock plant in September, sold 154 during October.