Cowles media heir Kingsley H. Murphy Jr. yesterday announced plans to sell his 17 percent stake in Cowles Media Co., publisher of the Minneapolis Star and Tribune newspaper, raising the possibility that it might be sold along with the Des Moines Register and Tribune Co.

Both newspaper companies are owned by members of the Cowles family.

Last week, Dow Jones & Co., publisher of The Wall Street Journal, and several Iowa businessmen, including two former Dow Jones employes, made a $112 million offer to buy the Register and Tribune Co.

Murphy sent a letter to Cowles Media Co. directors yesterday explaining that he was taking independent action to maximize the value of his investment by retaining Morgan Stanley & Co. Inc. and the New York law firm of Skadden, Arps, Slate, Meagher & Flom as advisers. In the letter, Murphy said Morgan Stanley believes that Cowles Media Co. is worth at least $350 million.

Meanwhile, the Register and Tribune Co.'s investment banker, First Boston Corp., began evaluating the $100-a-share, $112 million offer from Dow Jones. David Kruidenier, Register and Tribune chairman, said no action will be taken until First Boston renders its opinion on the fairness of that offer. The Register and Tribune Co. publishes the Des Moines Register newspaper.

Murphy's decision to sell his 17 percent share of Cowles Media will influence First Boston's evaluation of the offer for the Register and Tribune, because the Register and Tribune owns 14.3 percent of Cowles Media. That block would increase significantly in value, perhaps to as much as between $40 million and $50 million, if Cowles Media were sold, because its full value would be realized.

Investment bankers said First Boston is likely to advise the Register and Tribune Co. to reject the Dow Jones offer as inadequate if it appears that the two companies will be sold.

The companies could be sold to the same buyer or in simultaneous transactions to separate buyers, investment bankers said yesterday.

They also said that several potential bidders are watching events closely. The Register and Tribune is Iowa's largest newspaper, with a circulation of 240,000 daily and 400,000 Sunday, and the Minneapolis newspaper's circulation is 362,000 daily and 579,000 Sunday.

Potential bidders for the Register and Tribune who also might be interested in buying the Minneapolis Star and Tribune include Capital Cities Communications, The New York Times Co., The Washington Post Co., The Tribune Co., Times Mirror Co., Gannet Co. Inc. and the Hearst Newspapers, according to the investment bankers.

Kruidenier criticized the $112 million offer from the Dow Jones group, which includes two of his own top executives at the Register and Tribune Co., saying that the buyout plan was designed without his knowledge.

"I regret the manner in which it was made," Kruidenier said. "It was totally unexpected and unsolicited, developed privately without my knowledge."

Two members of the Dow Jones' group -- Register and Tribune Co. President Michael Gartner, and Gary Gerlach, publisher of the Des Moines Register -- have been placed on paid leaves of absence pending the outcome of the offer.

The Register and Tribune Co. has tentatively scheduled a board meeting for Dec. 10 to decide on the $100-a-share offer. When the offer was made, stock in the privately held company was trading in the mid-30s.

Kruidenier is one of five people who control a voting trust set up in 1978 that holds 52.7 percent of the Register's voting stock. Any successful bidder for the Des Moines company presumably would have to win approval of the trust.

In addition to the Des Moines Register newspaper, the Des Moines Register and Tribune Co. owns several small newspapers, the NBC television affiliate in Honolulu, the ABC television affilliate in Moline, Ill., four radio stations, and 14.3 percent of Cowles Media Co.

In addition to the Minneapolis newspaper, the Cowles Media Co. owns the Rapid City Journal, Rapid City, S.D.; the Great Falls Tribune, Great Falls, Mont.; the South Idaho Press, Burley, Idaho; and a group of community papers in the Denver area. Cowles Media also owns a commercial printing business in the Minneapolis area, as well as real estate holdings.