CHICAGO (AP) _ A strike by three production unions against the Chicago Tribune was in its second full day today amid indications the walkout could extend into next week.

Negotiations between the newspaper and Chicago Typographical Union Local 16 broke off after about three hours Friday, with conflicting reports about the progress toward a contract agreement.

The nation's seventh-largest newspaper had not been struck since 1947, and continued to publish after the walkout, but in diminished size.

A special note to readers on the front page of today's editions said the newspaper would continue to publish, but may not be available in as many editions during some 24-hour periods.

News sections of Thursday's editions, before the strike, were 42 pages, Friday's were 24 as were today's.

Bill Boarman, local vice president, said after the talks broke off that the union had presented ''a comprehensive proposal,'' but it was rejected by the Tribune.

''It's clear to us that the Tribune wants to be non-union,'' he said.

Bob Dickey, the Tribune's marketing department manager, disagreed with the union's assessment of ''no progress.''

''There was some positive dialogue at the session,'' Dickey said. ''But the length of the strike is largely dependent on the men who walked off their jobs.''

More than 1,000 members of the Typographical Union, the Chicago Web Printing Pressmen's Union Local 7 and Chicago Mailers Union Local 2 went on strike Thursday night. A key issue in the walkout, union officials said, was the Tribune's insistence on forcing mandatory employee transfers to other positions.

Typographical local president David Donovan said the union agreed Friday to accept a form of mandatory personnel transfers, but also requested a plan to compensate employees who decide to reject such transfers in favor of early retirement.

Donovan said the two other unions had not been consulted directly on the offer.

Talks with the typographical union were to resume Wednesday afternoon, said Tribune spokeswoman Ruthellyn Roguski. Negotiators for the Tribune planned to meet Monday with representatives of the pressmen, she said, but no talks had been scheduled with the mailers,

The Tribune was relying on non-union workers to put out the newspaper on a schedule ''as normal as possible,'' said Ms. Roguski.

Editorial employees are not represented by a union at the Tribune.

The Tribune has a daily circulation of 775,000 and a Sunday circulation of 1.2 million.

Midwest editions of USA Today, printed at Freedom Center, went out Friday and will go out again Monday, said a spokesman for the nationwide newspaper in Arlington, Va., who asked that his name not be used.

The three production unions had been negotiating separately for 1,125 workers.