Burroughs Corp. yesterday said it was prepared to sweeten its $70-a-share, $4.1 billion bid for computer rival Sperry Corp.

The announcement follows Sperry's rejection of the Burroughs bid on Wednesday as "wholly inadequate" and the company's proposed counteroffer of an $80-a-share buyback.

In a letter sent yesterday to Sperry Chairman Gerald J. Probst, Burroughs Chairman Michael Blumenthal said: "While we believe that $70 per share . . . is a full and fair price, we are prepared to negotiate a meaningful increase in our price."

A Sperry spokesman declined to comment on the letter. Investment banking sources indicated that the Sperry board of directors met yesterday afternoon.

In the letter, Blumenthal expressed the desire to "commence negotiations immediately," and added that Burroughs did "not want to see Sperry dismembered or engaged in complex financial maneuvers to frustrate what we believe to be an ideal marriage."

Sperry stock edged 37 1/2 cents a share higher to $73.25 in New York Stock Exchange trading, while Burroughs dropped 87 1/2 cents a share to $59.

A merger of Burroughs and Sperry would create the world's second-largest computer company, displacing Digital Equipment Corp. from its place behind International Business Machines Corp.

Analysts believe the Sperry counteroffer is designed to boost the value of Sperry stock closer to a $75-a-share price.