Contract talks resumed yesterday between the U.S. Postal Service and negotiators for two unions representing 579,000 workers whose wage-benefits contract expires today at midnight, officials said.

Talks broke off acrimoniously last Wednesday with a walkout by representatives of the two unions over the Postal Service's refusal to disclose terms of its tentative contract with a smaller union, the Mail Handlers Union.

Vincent R. Sombrotto, president of the National Association of Letter Carriers, and Moe Biller, president of the American Postal Workers Union, yesterday said their unions are determined to negotiate a new three-year agreement by midnight.

"We are very optimistic" about arriving at an agreement "at least by the deadline," said Bob Becker, a Postal Service spokesman.

No disruption of mail service is anticipated even if an accord is not reached, because federal law prohibits strikes by postal workers. An arbitration procedure would be used if there is no agreement.

At issue when the talks broke off were provisions of the tentative Mail Handlers agreement that the Postal Workers Union contended would transfer workers from its jurisdiction to the Mail Handlers Union. The Letter Carriers walked out in solidarity with the Postal Workers.

Talks resumed noon yesterday after the signing of a memorandum of understanding on that issue.

Becker said the two unions' concern over abridgment of the Postal Workers' jurisdiction "has been resolved to everyone's satisfaction," following several days of talks.

The two large unions made their formal money proposal last Tuesday, asking for wage increases of about 6.8 percent annually for three years, a shorter time to rise to top pay levels, and improvement in medical benefits and other provisions.

Postal negotiators had not made a formal response when the talks broke off Wednesday, and declined to publicly discuss their offer.

The Mail Handlers Union, a division of the Laborers' International Union of North America, represents 50,000 workers nationwide who perform such tasks as loading and unloading mail sacks and packages and sorting bulk mail.

The Postal Workers Union represents the nation's 346,000 post office clerks, maintenance workers, motor vehicle operators and special delivery messengers. The Letter Carriers union represents the 233,000 letter carriers who deliver mail in cities.