Tesdata Systems Corp. reported its first quarterly profit this year, although the earnings were sharply below last year's figure.

Tesdata, a McLean-based maker of computer and data communications measurement equipment, said it earned $124,000 (10 cents a share) in the third quarter, down from $648,000 (50 cents) a year ago. Sales fell to $3.6 million from $5.6 million.

The company said the results were attributable to changes in management and efforts to reduce costs that began in April. Tesdata has lost $1.2 million so far this year on revenues of $10.8 million compared with a loss of $433,000 on sales of $13.8 million in the first nine months of 1981.

Biospherics Inc. also reported a third-quarter profit that reduced the company's loss for the year. The Rockville-based environmental technology firm earned $60,453 (3 cents a share) in the quarter on sales of $1.4 million. A year ago, it earned $44,113 (2 cents) on sales of $1.6 million.

So far this year, the company has lost $32,540 on sales of $3.8 million, compared with a profit of $101,255 (5 cents) on sales of $4.6 million a year ago. Company President Gilbert V. Levin attributed the improved third-quarter performance to cost-cutting efforts and projected that the company would show a profit for the year.