Retail sales in metropolitan Washington soared 16 per cent in March from the same month last year, reflecting a healthy Easter shopping season and the largest year-to-year gain for local retailers in more than a year.

According to the Commerce Department, sales at all area retail establishments totaled $858 million in March compared with $738 million in the 1976 period. Nationwide, the gain in sales for March was 11 per cent.

The March gains followed lackluster increases over the previous year that ranged from a low of 4 per cent in January to 10 per cent in December and February. For all of 1976, area retail sales grew by 8 per cent, a modest level of increase that local retailers attributed to the election year and changing administrations.

For the first quarter of 1977, area retail sales were up 11 per cent from the same period a year ago at $2.35 billion - again somewhat better than the average nationwide retail sales gain of 7 per cent.

Sketchy data provided by the Census Bureau, however, indicates that downtown Washington retailers may not be participating in the 1977 gains.

The Commerce Department surveys three large downtown retailers for its monthly sales reports - generally thought to be Woodward & Lothrop, the Hecht Co. and Garfinckel's, all along F Street NW. At the three stores, downtown, sales were off 2 per cent in March from the same month in 1976 to $6.8 million. And, for the first quarter of 1977, downtown sales at the three stores also were down 2 per cent to $18.8 million.

At seven other central business districts surveyed each month, sales were flat or down in the first three months by as much as 13 per cent in Baltimore and Philadephia. For the month of March alone, however, sales in downtown department stores were up in Chicago, Cleveland, Los Angeles, Oakland and Pittsburgh.

For all department stores in metropolitan Washington, March sales totaled $107 million, a gain of 15 per cent from the same month last year.*