Computer Sciences Corp. said yesterday it was ending exploratory talks about purchasing Planning Research Corp. and Advanced Technology Inc., two Northern Virginia government contracting companies owned by Black & Decker Corp.

Most companies refuse to publicly discuss their acquisition activities unless and until a formal purchase agreement is announced. But a CSC spokesman said the El Segundo, Calif.-based computer company took its unusual step because of reports that its directors were reviewing a proposal to buy the two subsidiaries of the Towson, Md.-based power tools manufacturer.

"We had several meetings that were exploratory in nature," William R. Hoover, chairman and president of CSC, said in a statement issued by the company. "Our board of directors was informed but, contrary to recent rumor, no proposal was presented for approval and, of course, no offer was made to Black & Decker."

Sources at Computer Sciences, Planning Research and Advanced Technology say the discussions never went very far. "There was some discussion and interest but it never crystallized," said one CSC official. He said the talks lasted only about two or three weeks.

Black & Decker, which acquired PRC and ATI last year when it purchased Emhart Corp., a Connecticut conglomerate, has been trying to sell the two high-technology government contracting companies for more than a year. Black & Decker recently initiated a reorganization of PRC and ATI in an effort to make them more attractive to potential buyers.

A Black & Decker spokeswoman said other parties have expressed interest in PRC and ATI, although she declined to identify the parties or characterize the discussions. Sources at both ATI and PRC said no formal offers have been made for the two companies.

CSC's statement comes on the heels of PRC's recent loss, to Cincinnati Bell Information Systems Inc., of two major General Services Administration contracts to provide technical support to business computer systems at federal agencies. The contracts will run for four years and are worth a total of $130 million.