Washington area retail sales for the first 10 months of last year were up a sluggish 7 percent from the same period in 1978, but downtown Washington department stores posted the best year-to-year gains in more than a decade.

According to the Commerce Department, sales of all metropolitan retailers -- including food stores, auto dealers, restaurants and general merchandise units -- totaled $10.86 billion for the January-October period of 1979. The 7 percent rate of increase here trailed the nationwide average gain of 10 percent.

However, sales in October alone were at the highest level of the year in the Washington area -- $1.18 billion, an increase of 10 percent over the same month in 1978 and close to the U.S. average gain of 11 percent.

With retail goods prices up 8 percent or more from the same period a year earlier, the recent sales data indicate that levels of actual merchandise transactions are about unchanged, with inflation accounting for the increases.

Major department stores in downtown Washington, meantime, reported a strong 10 percent increase in sales for the first 10 months of 1979 over the previous year to $78 million.

Sales at the city's big general merchandise retailers -- those remaining downtown are Woodward & Lothrop, Hecht's and Garfinckel's -- declined overall on a steady basis during the past decade after the 1968 rioting and a proliferation of suburban retail malls.

The sales decline continued through 1976 but then began to level off. A resurgence of downtown sales in the past year has been attributed partially to increasing use of the expanding subway system which included an entrance into the basement of the Woodies flagship store and a separate station near the Hecht Co. store.

With more downtown workers commuting by public transportation or car pools in the wake of soaring gasoline prices, retailers also have reported an increase in daytime shopping in the city.

For the area as a whole, department store sales in the first 10 months of 1979 were up 8 percent -- somewhat less than the downtown D.C. increase.

Of eight central city retail centers surveyed monthly by the Commerce Department, Washington had the largest increase for the January-October period last year. Department store sales were up by 7 percent in downtown Los Angeles and Oakland and up by 2 percent in Chicago.

In downtown Baltimore, with fewer department stores in business, sales plummeted 14 percent from 1978. Sales fell by one percent in downtown Philadelphia and were unchanged in Cleveland and Pittsburgh.