Giant and Safeway supermarkets are locked in a dispute with their 500 Teamsters Union truck drivers that could lead to a strike at midnight tonight, according to company and union officials, who said the deadlock revolves around a company plan to cut pay for newly hired employes and schedule more drivers on Saturdays.

A key issue in the negotiations, under the supervision of federal mediators, is a proposal for a "two-tier" wage scale in which new drivers would start at about $8 an hour and earn a maximum of about $10 an hour. Veteran drivers currently earn $13.87 an hour at Giant and $14 at Safeway and would remain on a higher pay scale.

Such dual pay systems, strongly resisted by unions, have become increasingly popular in many industries as a means of reducing labor costs without cutting the pay of current employes. The United Food and Commercial Workers (UFCW), representing more than 10,000 Giant and Safeway employes, accepted a two-tier system last year, but the Teamsters have threatened a strike if the proposal is not dropped when the contract expires tonight.

In the event of a strike, both supermarket chains are expected to continue operating, although Teamster officials are hopeful that UFCW's unionized grocery clerks and warehouse workers would honor picket lines and thwart the chains' efforts to stay open. UFCW officials were not available for comment.

"I am hoping we do not have a strike and can reach agreement," said Phillip Feaster, president of Teamsters Local 639, whose drivers serve 150 Safeway and 130 Giant stores. Feaster added that the union is willing to compromise on issues other than the dual pay scale.

"We believe the American work force was built on everybody getting the same pay for the same day's work, and that is an issue we will take a strike over," Feaster said. "I have said we will fight it to the last straw and I mean it."

Giant's vice president for labor relations, Roger Olson, said yesterday the lower pay scale for new employes "is the only way other than reducing wages of current people to average down our wage costs. That allows us to compete" in the highly competitive Washington-Baltimore supermarket industry.

"This is a sign of the times," Olson said. "We are competing against competition that is paying considerably lower rates" of pay. He cited Peoples Drug Stores, which pays Teamster drivers $9.50 an hour, and Richfood, a Richmond distributor, which pays $8.26, as examples of the wage gap that Giant seeks to close.

Safeway labor officials, who were meeting late yesterday with the Teamsters at the Federal Mediation and Conciliation Service, were not available for comment. But a company spokesman said, "We are optimistic . . . and we plan to keep operating" even in the event of a strike. Giant's negotiations were to resume this morning at the FMCS headquarters.

Another disputed proposal is a company move to raise a contractual limit on the number of drivers scheduled to work Saturdays. Most drivers work Monday through Friday and receive bonus pay when asked to work Saturdays, but the new plan would shift more drivers to a Tuesday-Saturday schedule.