Media magnate Rupert Murdoch, being forced by federal rules to sell his tabloid newspapers in New York and Chicago so he can purchase a group of television stations from Metromedia Inc., yesterday put the Chicago Sun-Times up for sale. But there was no word on his plans for the New York Post.

Murdoch could not be reached for comment.

The announcement that Murdoch would sell the paper he bought for $90 million 16 months ago came in a memo posted in the Sun-Times' newsroom. "Yes, it's true that Rupert Murdoch has decided to sell the Sun-Times," said the memo, over the signature of Publisher Robert Page, who is traveling in China. "He has determined that he will comply with FCC regulations which now prohibit cross-ownership of newspapers and TV stations in the same market."

Rumors circulating in Chicago had the Toronto Sun and one or two local groups considering bids for the newspaper, which has a daily circulation of about 650,000.

Murdoch had reportedly considered asking for a waiver of a Federal Communication Commission regulation that prevents ownership of both a newspaper and broadcast stations in the same city, but then decided to comply with the rule and sell the newspapers. The rule creates a conflict for Murdoch in New York and Chicago because of his planned purchase, with oilman Marvin Davis, of Metromedia Inc.'s seven television stations, including WFLD-TV in Chicago and WNEW-TV in New York, for $2 billion. Murdoch and Davis have already agreed to sell one of the stations, WCVB-TV in Boston, to Hearst Corp. for $450 million. That would eliminate another possible cross-ownership conflict with Murdoch's Boston Herald-American.

Murdoch's purchase of the Sun-Times in November 1983 was much criticized in Chicago, provoking fears that he would turn the paper -- long the liberal champion of the city's working-class neighborhoods -- into a copy of his politically conservative and journalistically flashy New York and Boston tabloids. Those fears have not really come true; although the Sun-Times editorial page has become somewhat more politcally conservative, the paper overall is only a bit more racy than it was before. Murdoch's purchase did, however, cause the defection from the paper of dozens of Sun-Times staffers, many of whom went to the rival Chicago Tribune.

Analysts have said that Murdoch might have some trouble selling the New York and Chicago papers because the two markets are highly competitive and the papers are not wildly successful -- particularly the Post, which is believed to be losing about $10 million a year despite circulation of about 1 million papers a day. The Sun-Times was seen, however, as the slightly more salable of the two, because it has been turning a slight profit.

The biggest winner from Murdoch's plans to sell the Sun-Times, and his expected plans to sell the Post, is the Tribune Co., the Chicago company that publishes the Chicago Tribune and New York Daily News. Murdoch has been seen by analysts as providing those papers with stiff competition that might not come from new ownership. The Tribune Co. also owns broadcast stations in both cities, but is exempted from the federal cross-ownership rules by a grandfather clause.

Tribune Co. officials had no comment last night on Murdoch's plans.