After reporting its best year ever in 1976. Drug Fair yesterday said sales for the year ended June 30 were virtually flat, and earnings declined by 30 per cent.

Drug Fairs after-tax profit was $2.9 million ($1.65 a share), down from $4.2 million ($2.37) the previous year. Despite the opening of several new drop stores and continued diversification with the opening of additional Wrangler Wranch units, the chains sales for the year increased only $11 million, to $229.4 million.

Fourth quarter results mirrored the year's performance. Sales increased from $56.7 million to $57.8 million, while profits dropped from $1.8 million to $1 million.

Nothing that the previous year had been the best ever for the chain. Drug Fair president Milton L. Elsberg blamed the downturn on competitive pressure in the chan drug business and a shift in consumer spending to high-ticket durable items and "away from the things we sell."

Elsburg said Drug Fair recently opened new stores in Ocean City and Deer Park, Md., and by year-end will add Wrangler Wranch units in the Skyline shopping center in Falls Church, in Chillum, and in Chambersburg, Pa.

Biospherics Inc., a Rockville-based specialist in waste water-treatment reported an increase in revenues to $1.68 million with profits of $398,000 (55 cents a share) for the six months ended June 30.

The results compared with revenues of $1.04 million and earnings of $123,000 for the same period a year ago. Sales of a single line of pollution control instruments produced most of the gain - $497,000 in added revenues and profits of $324,000 (45 cents) - said Williard M. Hillegeist vice president for finance.

Washington Real Estate Investment Trust said a $1.6 million gain on the trade of an apartment house for an office building boosted its net income for the second quarter to $2.2 million.

That exchange provided $1.08 of WRIT's $1.48 per share earnings, compared to 43 cents a share for the same period last year when the trust earned $629,000.

The company declared its regular quarterlydividend of 44 cents per share payable Sept. 30 to shareholders of record as of Sept. 7.