Tesdata Systems Corp., a Northern Virginia firm that is the nation's leading producer of equipment that monitors the efficieny of computers, plans to "go public" with an initial offering of 500,000 shares of common stock.

The proposed offering, expected next month would be the first new issue by a significant Washington area corporation since the early 1970s, when such firms as Hechinger Co., W. Bell & Co., Carl M. Freeman, the Washington Post Co. and several real estate trusts sold public shares.

A subsequent stock market decline and recession put a damper on efforts by established or young companies to raise capital through equity offerings.

Of the Tesdata shares to be offered, 200,000 will come directly from the McLean company; 140,000 from the firm pursuant to the exercise of warrants; and 160,000 shares from selling stockholders.

'Anunderwriting team of investment companies, Alex Brown & Sons of Baltimore and Hambrecht & Quist of San Francisco, estimated the initial offering price would be in a range of $13 to $16 per share.

Tesdata was incorporated in 1970 and commenced business operations in January 1972 under founder and chairman Thomas E. Stone.

Stone had set up Computer Learning and System Corp. (later Clasco, Inc*.) here in 1967, to engage in computer and educational services. As troubles beset small computer companies starting in 1969, business suffered and the systems division was sold to Stone and other Clasco stock-holders in 1972.

Current systems marketed by Tesdata are designed to save money for business, government and organizations that own or lease computers, by locating bottlenecks in data processing . Like effiency experts that study a company's personnel. Tesdata equipment tests the effectiveness of the co*mputer age.

Tesdata recently completed its largest contract to date-the installation of 11 computer monitors at the Social Security Administrations's computer center inBaltimore, where more than 100 computers process more than half a billion entries-making mistakes in the process, according to studies of Social Security errors over the past two years.

The four other largest Tesdata customers currently are Lufthansa German Airlines, Chesapeake & Potomac Telephone Co., Deere & Co. and Bank America Corp., owner of the country's largest Business Machines Corp. uses Tesdata monitors in Germany to test customer's machines and other customers include American Airlines, General Electric., and New York Life Insurance Co.

According to previously confidential data contained in a preliminary prospectus, filed Tuesday with the Securities and Exchange Commission, Tesdata last year had profits of $1.3 million (1.44 a share, fully diluted), up sharply fro $316,000 (41 cents) in 1975 and $63,000 (9 cents) in 1974.

Sales last year rose to $10.4 million from $6.1 million in 1975 and $5.1 million in 1974.In an interview last fall, Stone forecast 1977 sales of $16 million but the preliminary prospectus states that "future increases in volume are not expected to result in similar percentage increases in profit margins."

Tesdata said it plans to use money raised in the public offering to pay off $629,000 of bank debts and other obligations connected with a recent acquisition and in development of its key computer monitor product; to support expanded marketing and product development; and to provide capital for development or acquisition in related fields.

The McLean firm last month accquired Inmet, Inc., a Florida manufacturer of systems that measure computer performance, including an environmental system for a Florida Power & Light Co. nuclear plant. Tesdata paid $75,000 in cash and assumed liabilities of $335,000. Inmet had 1976 revenues of $400,000 and operated at a small loss.

Among other factors revealed in the registration statement:

Upon completion of the proposed offering, there will be 637,538 shares of common stock owned by current stockholders, or 56 per cent of the total then outstanding. About 201,000 of these shares will be eligible for sale 90 days after the public offering.

Tesdata has not paid any dividends and has established no policy regarding payouts.