Eighteen months after Rupert Murdoch's purchase of the peppery Sun-Times stunned Chicago, the Australian media magnate has surprised the city anew with a move that may force him to sell or close the tabloid he fought so hard to acquire.

Chicagoans had no advance warning of Murdoch's plan to pay more than $2 billion to buy television stations from Metromedia Inc. in six cities, including Washington, New York and Chicago. Metromedia said today that it has agreed to sell him the stations. Such a purchase apparently would force him to yield ownership of the Sun-Times and the New York Post to comply with Federal Communications Commission regulations limiting media ownership.

Three weeks ago, Murdoch visited here and reassured major Sun-Times advertisers of his long-term commitment to the paper. Sun-Times publisher Robert E. Page was on a long-planned vacation in the Far East when the latest development broke.

Page told his staff in a message today that "it cannot be said with certainty" whether the paper will be sold. He emphasized that the full sale transaction could take two years. Installed as publisher by Murdoch soon after the paper changed hands last year, Page is reportedly on his way back to Chicago.

Details of Murdoch's decision to move heavily into television remain to be disclosed. In weekend interviews, Murdoch, 54, affirmed his intention to become a U.S. citizen, but declined to say anything about possible divestiture moves.

Media analysts said they expect Murdoch to put the Sun-Times and the New York Post up for sale to meet an FCC rule barring a newspaper publisher from owning more than 5 percent of a broadcast outlet in the same city.

Metromedia's station in Washington is WTTG, Channel 5.

Several newspaper executives elsewhere said they believe that Murdoch may close the Post, whose losses run about $9 million annually.

"If you had your choice of having a money-loser the Post that sold a million copies daily, or a money-winner Metromedia's WNEW, Channel 5, in New York , watched by million daily, which would you want?" one executive asked.

The tabloid Post's major rival is the Daily News, owned by the Chicago Tribune, powerful adversary here of the tabloid Sun-Times.

At least one editor suggested that Murdoch may be interested in shutting down the Sun-Times. He acquired the 44-year-old daily in a controversial $100 million deal with the Marshall Field family, which spurned the last-ditch efforts of a group of local businessmen to keep the paper's ownership here.

Uncertainty about Murdoch's plans today permeated the fourth-floor newsroom of the Sun-Times, the nation's 10th largest circulation daily, selling about 639,000 copies. One reporter said the mood was one of "simply waiting it out."

Metromedia's Chicago television station is WFLD, Channel 32, which had advertising revenues of about $45 million last year and profits of about $20 million, according to local reports. The Sun-Times, whose Sunday circulation is 690,000, is thought to have made a profit of several million dollars last year.

After Murdoch took over, the paper incurred substantial circulation losses, which have not been fully restored. Dozens of experienced reporters and editors left after Murdoch took over, and many jobs have not been filled.

The most celebrated departure was that of popular local columnist Mike Royko, who moved to the Tribune and delivered a blistering denunciation of Murdoch.

Sale of either the New York Post or the Sun-Times would be a substantial victory for the Tribune.

In his statement from abroad, publisher Page said, "The acquisition is a very complex deal, and it cannot be said with certainty whether, if ever, the Sun-Times would be sold as part of an overall transaction satisfactory to the FCC." Page said there may be grounds to appeal or work around the FCC rule because the rival Tribune owns a major Chicago television station -- WGN, Channel 9, also available on cable systems nationwide and overseas. It is Chicago's most profitable station, with advertising sales of nearly $90 million last year.

"It is conceivable the government would permit Murdoch to own both Channel 32 and the Sun-Times, in view of the market position of the Tribune Co. and its ownership in this city of a paper and TV and radio stations," Page said.

A local FCC official said the Tribune's cross-ownership predates the FCC rule, however, "and has been handled in a grandfather clause many times over."

Informal discussions have been initiated among some businessmen whose effort to outbid Murdoch for the Sun-Times failed. "There has been some talk, nothing more," one source said. "There may be interest." But the prospect of serious inquiries by any of these men await word on Murdoch's plans.

"There is a lot more money in television," said Bill Suter, a vice president for media stocks with Merrill Lynch & Co. Inc.

John S. Reidy, analyst for Drexel Burnham Lambert Inc., ruled out any possibility that Murdoch would keep the Sun-Times and the Post and sell the television stations here and in New York.