The Baltimore Sun newspapers were sold yesterday to the Times Mirror Co. for $600 million, only one day after Baltimore became a city with one newspaper company because The Baltimore News American folded.

Ending a 149-year history of family ownership of the Sun papers, the Los Angeles-based Times Mirror company agreed to purchase theA. S. Abell Co., which owns television stations in Baltimore and Richmond, in addition to two newspapers, a spokesman for Times Mirror said.

"The timing of our acquisition and the closing of the Baltimore News American is coincidental," said Donald S. Kellerman, Times Mirror's vice president for public affairs. "We did not have any idea it was going to close yesterday [Tuesday]."

The addition of the morning Sun, founded in 1837 as a penny sheet by Arunah Sheperdson Abell and the Evening Sun, the paper of H. L. Mencken, brings to nine the number of newspaper properties owned by the Times Mirror Co., which include the Los Angeles Times, New York's Newsday and the Dallas Times Herald. The company also owns television and cable systems and publishes books and magazines.

The Sun papers -- which operate with separate news staffs but share a printing plant, advertising and circulation departments -- have a combined daily circulation of almost 357,000. The papers have won 10 Pulitzer prizes.

"We're very excited," said Robert F. Erburu, chairman and chief executive officer of Times Mirror. Erburu said he first approached William E. McGuirk Jr., chairman and president of Abell, about selling on May 13. However, he said the company had been interested in the Sun papers for several years. "We just decided to see if we could persuade them."

The Abell Co. is one of the first big media companies in several years to be sold without first opening a bidding process. Several newspaper publishers have decried the recent bidding wars for papers such as those in Detroit and Louisville, which sent prices soaring and priced many buyers out of the market. In the most recent newspaper auction, the Louisville Courier-Journal and Times sold for about $315 million to the Gannett Co. Inc.

However, Times Mirror has a record of negotiated transactions with newspapers such as Newsday and the Hartford (Conn.) Courant and prefers to buy properties that way, according to one Times Mirror executive. Officials from both companies said there were no other offers for the papers.

"I am delighted that the Baltimore Sun will be joining Times Mirror," McGuirk said. "There is no finer newspaper organization."

Erburu stressed that the Sun newspapers would continue under their present management. "One of the happy parts of this for us is that Reg [John Reginald] Murphy is publisher and will continue to be the publisher," Erburu said. "Our policy is one of a very strong degree of local autonomy. Our publishers have a lot of leeway to operate."

"I wouldn't encourage you to believe that there are going to be major changes," Murphy, president and publisher of the Baltimore Sun papers, said late yesterday. "I consider Times Mirror to be the best newspaper company in the country."

One of the things Times Mirror will not be able to keep is the Abell's Baltimore television station, NBC affiliate WMAR, since FCC rules do not allow a company to acquire a newspaper and a television station serving the same market.

While the Abell Co.'s sale has been unanimously approved by the boards of directors of both companies, it is subject to negotiation of a final agreement and approval by the Abell Co. shareholders and the government. More than 80 percent of the Abell stock already is committed to the sale, according to a Times Mirror spokesman.

"The Sun people are very lucky that they have had great ownership through the years and that they wind up with one of the classiest companies in the business," said Donald E. Graham, publisher of the competing Washington Post. "We're glad to have Times Mirror down the street."

David Laventhol, Times Mirror's senior vice president for eastern newspapers who will become president of the company in January, said the Evening Sun would continue to operate as a competing daily.

"When the telephone calls began to come in today for subscriptions in the wake of the News American's closing the ratio was 6-to-1 in favor of the Evening Sun," Laventhol said. "It is a vital newspaper."

By the end of the day, he said, the Sun papers would have "thousands" of new subscriptions from old News American subscribers.

News of the sale appeared to have been tightly held in the paper's own newsroom, according to members of the staff, who gave the following account:

At about 4:35 p.m., one reporter heard a rumor of sale from a friend in Los Angeles. He passed it along to staffers gathered around the coffee pot in The Sun newsroom.

At about 4:45 p.m., James I. Houck, the managing editor of The Sun, confirmed to the staff that it was true.

Reporters and editors from both newsrooms then began to gather. The newsroom is being renovated -- new offices, floors and desks -- and some of the crowd stood on boxes.

The audience was more stunned than angry, and the questions reflected uncertainty about the future. Houck was asked about union agreements, about changes, about comparisons to Times Mirror-owned papers in Hartford and Denver, about rumors that Murphy had attempted to buy the paper himself.

The Abell Co. had revenue of $204.8 million in 1985. Times Mirror's sales reached $3 billion last year.

Times Mirror disclosed on Tuesday that it was buying Washington's National Journal, a weekly magazine that covers politics and government in the nation's capital.