Mark J. Meagher, who resigned as president of The Washington Post Co. in January, today was named president of Penthouse International Ltd., which publishes the men's magazine Penthouse.

Until today, Bob Guccione, who founded both the magazine and the company in Britain in 1965, had been both chairman of the board and chief operating officer of the company. Guccione will continue as chairman. In 1970, Guccione introduced Penthouse to the United States as a more risque alternative to the nation's leading men's magazine, Playboy.

Over the decade Penthouse circulation has expanded sharply. It sells about 4.3 million copies a month, while Playboy sells 5 million.

In addition to Penthouse, the company publishes two digest-sized, sexually oriented magazines -- Forum and Variations -- as well as a science magazine, Omni.

The company has produced one movie, Caligula, and is making another one about Catherine the Great, the empress of Russia. It also is building a gambling casino in Atlantic City, although construction has been halted while the company seeks additional financing.

Guccione, in a statement, said that Meagher will "be responsible for all strategies, the implementation of long and short-term goals and staffing for all Penthouse International properties, including the four magazines, the film and television production companies, the Atlantic City casino operations and all other developing enterprises in progress at this time."

Meagher joined The Post Co. in 1970 as vice president for finance. In 1974 he was named general manager of The Washington Post newspaper and in 1976 became president of the company's newspaper division, which includes The Trenton Times and The Everett Herald in Washington State. In 1977 Meagher was named president of the parent company.

Meagher continues as a director of The Washington Post Co.

Meagher said that he decided to move to Penthouse both because he finds Guccione "the most creative, exciting person I've ever dealt with" in communications and "because of the professional challenge."

In a telephone interview Meagher said that Penthouse, like many companies that have grown rapidly, "has not formalized the management processes and controls" it needs to continues to grow. He said that it will be his job to bring "the business side of Penthouse" up to the editorial side. Penthouse is a privately owned company.