After fears mounted that National Public Radio would be unable to meet tomorrow's payroll, the Corporation for Public Broadcasting yesterday provided a temporary bail-out in the form of a $500,000 interest-free loan.

"The loan is principally to help us with the cash-flow problem, not only the payroll," said Ronald C. Bornstein, the acting chief operating officer of NPR. The network, with a payroll of about 350, is scrambling to find ways to eliminate a $5.8 million deficit.

Bornstein said he learned of the possible inability to meet the payroll and other expenses last week, his first week as the executive in charge of day-to-day management operations. At a meeting with NPR's department heads Tuesday, Bornstein had warned there might be problems meeting the payroll. But yesterday morning he issued a memo to the staff saying the immediate cash problems were solved.

"We are pleased and appreciative that the Corporation has provided such assistance," said Bornstein from Madison, Wisc., where he works as the acting chancellor of the University of Wisconsin-Extension. He has taken over the leadership of the beleaguered network from Frank Mankiewicz, the president for the last five years, who resigned last week.

Lawyers for NPR and CPB, which aids in funding public broadcasting, along with CPB president Edward Pfister, met for more than four hours in Washington working out the details of the loan. Pfister could not be reached for comment.

The network's deficit, which Mankiewicz orginally estimated at $2.8 million, is now described as put at $5.8 million, an estimate that might be further revised after audits being made by both NPR and the General Accounting Office. The results could change the programming format of the award-winning network.

For the news and public affairs departments, the working projections for the fiscal year 1984 budget range from $3.9 million, which NPR management said earlier in the week was the worst-case scenario, to $.6 million, the estimate for the arts and performance division range from zero to $400,000. In the first round of cuts in March, that division lost "The Sunday show" and is scheduled to lose "Jazz Alive." The network has also eliminated the news portions of "NPR Plus."

Bornstein has said he is committed to the largest budget possible for news and public affairs. "The last thing we would want to impact on is on-air procuction," he said.