Cowles Media Co., the Minneapolis-based publisher of the Minneapolis Star and Tribune newspaper, made it clear yesterday that the company is not for sale.

David Kruidenier, chairman of Cowles Media, and John Cowles Jr., a major stockholder and former chief executive officer, told a news conference the family's continued ownership has been ensured as the result of recent negotiations among Cowles family members extending the family's voting trust through the year 2000. They said the new trust agreement includes the stock of third- and fourth-generation members of the family, giving the trust more than 50 percent of the company's stock.

The announcement was made exactly three weeks before the directors of The Des Moines Register and Tribune Co., publisher of Iowa's largest daily newspaper, are scheduled to choose a new owner for the newspaper and other properties. The Des Moines Register and Tribune Co. also is owned by Cowles family members. Kruidenier is chairman of both the Des Moines and Minneapolis companies.

Kruidenier has said that almost all of the nation's major newspaper companies have expressed serious interest in buying The Des Moines Register, which has a daily circulation of about 240,000.

The company's investment banker, First Boston Corp., will solicit bids from interested parties. While no final decision will be made until the bids are in, it appears that The Des Moines Register and Tribune Co.'s assets will be divided up and sold to several buyers.

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

It would appear that the value of the Register Co.'s 14.3 percent block of stock in Cowles Media would be lower as a result of yesterday's announcement because it makes Cowles Media less likely to be a takeover candidate in the near future.

However, before directors decided to sell The Des Moines Register and Tribune Co., that company had a voting trust that was designed to ensure continuity of ownership. But when Dow Jones & Co., publisher of The Wall Street Journal, and several Des Moines businessmen offered $112 million for The Des Moines Register and Tribune Co. on Nov. 5, pressure to sell prevailed over that voting trust.

Kruidenier has said potential buyers include The New York Times Co., The Washington Post Co., Times Mirror Co. and Knight-Ridder newspaper. Other potential bidders include Capital Cities Communications, Dow Jones & Co., The Tribune Co. of Chicago, Gannett Co. Inc. and the Hearst Newspapers.

Representatives of The New York Times, Gannett and The Tribune reportedly are visiting Des Moines this week to learn more about the company.

Although no final decision has been made about what to do with the 14.3 percent block of Cowles Media owned by The Des Moines Register and Tribune Co., the options include selling the block to Cowles Media, selling the block to the highest qualified bidder and distributing the block as a dividend to stockholders of The Des Moines Register and Tribune Co.

Kruidenier and John Cowles Jr. reaffirmed their confidence in Cowles Media Co.'s management yesterday. They said the Twin Cities are an outstanding market with excellent growth prospects for the Minneapolis Star & Tribune newspaper. "We believe that confirming a long-term commitment to the continuity of independent ownership and professional management is very important . . . ," they said. "To ensure continuity of ownership, we have agreed to take the steps necessary to extend the voting trust for an additional 10 years, through the year 2000, and additional members of the Cowles family, from the third and fourth generations, have agreed to join the voting trust, which will increase voting shares to more than 50 percent. We will both continue as trustees of the voting trust.

"We both, together with other members of the family, are confident that these steps provide the stable framework on which the company can grow and prosper, meet the financial goals necessary for that growth, and maintain the standards of excellence long asssociated with the Cowles name."

Previously, the voting trust had only 44.6 percent of Cowles Media Co.'s stock.

It was also announced yesterday that John Cowles III, son of John Cowles Jr., will join Cowles Media Co. as director of planning. One source said he believed the two announcements were linked, suggesting that some family members agreed to support the extension of the Cowles Media voting trust as long as John Cowles III was given an important role in the company's management.