Planning Research Corp. -- the world's largest professional services, on consulting, firm -- announced a major reorganization plan yesterday designed to emphasize new markets for the company during the 1980s, particularly the computer software field.

"We've analyzed the whole business and said we're really in four businesses," John Toups, PRC's president and chief executive officer, said in an interview. "I think we are really restructuring ourselves to address the markets of the 1980s -- the computer software, energy and aspects of the defense markets."

In essence, the decision separates PRC into four corporate units: Commercial Information Systems, Government Information Systems, Architectural and Engineering Services, and Systems Engineering and Facilities Support.

Previously, only two groups -- the planning and architecture group and the information sciences and services unit -- reported to Toups and other members of PRC's top management. The two groups operated 17 branches of the company.

The restructuring will put additional emphasis on the commerical information systems business, which includes computer software and systems markets. Less than one-fourth of the District-based company's business today is in commercial software business, and Toups said he'd like to see that PRC field grow to about half the firm's work, a recognition of the company's desire to become less dependent on federal contracts.

The move is also part of a major top-level management study at PRC designed to reshape the corporation's mission. Both Toups and PRC Chairman Robert Sarnoff have said they feel the company has performed inadequately in recent years, in large part as a result of an inadequate coordination between PRC's various and diverse enterprises.

But the announcement comes at a time of rapidly increasing, if somewhat mysterious, heavy activity in the company's stock, which is traded on the New York Stock Exchange. In fact, the stock has been among the most active on the exchange in recent weeks and rose to 12 3/8 Monday, after being as low as 4 1/2 within the last year.