Personal income in the United States rose modestly in October while consumer buying declined slightly, according to a report released yesterday by the Department of Commerce.

The last time consumer buying dipped was in February. The 0.1 percent decline in consumer buying -- which includes purchases of goods and services but does not include debt payments -- followed a 1.7 percent jump in personal consumption spending in September.

The 0.6 percent increase in personal income is close to the average increase so far this year. Disposable income, what consumers have left after taxes, also rose 0.6 percent. Neither total personal income nor disposable income are adjusted for inflation.

Incomes have been rising faster than inflation for the last two years. Commerce Department chief economist Robert Ortner said that real, or after-inflation, income probably grew by about 0.2 or 0.3 percent in October. The real growth rate cannot be calculated until inflation figures are available later.

Personal consumption spending had been flat in July and August -- an indication that the economy had settled into a more subdued mode than it had for much of 1983 and 1984, when the economy was growing at a rapid rate.

Consumers are the major driving force in the economy.

Most of last month's decline in consumer buying occurred in the automobile sector and in the often-volatile furniture sector. Economists attribute little significance to month-to-month buying data. For example, the decline in automobile expenditures may have been attributable to a shortage of desirable models because of strikes in both the United States and Canada.

The Commerce Department's Ortner said there is little to worry about in the 0.1 percent decline in consumer buying last month. That decline had been foreshadowed by earlier reports that retail sales fell in October -- although personal consumption expenditures measure not just retail sales but consumer purchases of services.

Ortner said that the outlook for consumer buying, especially in the coming Christmas season, remains strong. He noted that personal incomes continue to rise and surveys of attitudes show that consumers remain confident about their economic future.

The Christmas season is the most crucial time of the year for retailers. As much as 40 percent of many stores' yearly sales occur in the period between Thanksgiving and Christmas. "We expect a very strong Christmas," Ortner said.

Although consumer buying has slowed this year, consumer savings is increasing. The savings rate -- the percentage of after-tax income consumers salt away -- rose to an annual rate of 6.5 percent in October, compared with 5.9 percent in September, when consumers upped their purchases markedly. For all of 1983, consumers saved only 5 percent of their disposable income.