Members of the union representing reporters, copy editors and advertising personnel of the Baltimore Sun and Evening Sun walked an informational picket line yesterday and voted to strike beginning at midnight tonight if a settlement is not reached in talks for a three-year contract.

The agreement between labor and management expired today but 300 members of Local 35 of the Washington-Baltimore Newspaper Guild agreed to extend the deadline for 24 hours. The Guild also represents commercial and newsroom personnel at The Washington Post, who have been working without a new contract for nearly a year.

The union and Sun management disagree over salary increases, health benefits and the existence of lower pay scales for employes who work in suburban areas producing zoned supplements.

The Guild has asked for 8 percent wage increases; the company has proposed a wage freeze in some categories and pay increases in most others that would amount to $10 a week in each year of the pact. The union wants medical benefits lost during 1984 negotiations restored; the company has asked that employes pay for medical benefits beginning, on the average, with an $8.72 weekly payroll deduction in the first year, $14.46 in the second year and $20.89 in the third year. The union also wants employes from suburban supplements to receive the same wages and benefits as those who work for other sections of the paper.

For readers, the only sign of trouble at the Calvert Street office was shown in stories published Tuesday: In the final edition of the morning paper, all reporters who wrote stories for the Maryland section refused to allow their bylines to appear. Sports, features and business news also were affected. When the afternoon paper was printed, only three reporters allowed their names on stories. National and foreign reporters are exempt from union membership and their bylines were retained.

Union negotiator Sandy Polaski said the "byline strike," which is expected to continue, was not a union action but "something called up by the ranks." According to those who withheld their bylines, the decision was based on the "company's unwillingness to bargain in good faith."

"The union doesn't regard what has been going on as bargaining," said reporter Michael Ollove. "The wage proposal on the table offered yesterday afternoon was the first one we've gotten. They are treating us contemptuously. If they were bargaining in good faith, none of this would be happening."

Sun management officials said yesterday the contract was extended by "mutual agreement" and they were hopeful for a settlement. Editorial and commercial workers last struck in 1978 for three days.

"We hope that the reason and trust exhibited so far will produce an acceptable solution," said Gerry Smolinski, public affairs manager for The Sun.

The Sun, a family newspaper for 149 years, was sold last May to the Los Angeles-based Times Mirror Co. A management spokesman said negotiations have continued to be handled by an in-house counsel.