Nancy Reagan's chief of staff, James S. Rosebush, announced yesterday he is leaving his $72,000-a-year post in January, but said in an interview that "I really haven't given much thought to what I'll do next."

Succeeding him will be Lee L. Verstandig, 47, undersecretary of housing and urban development who previously was at the White House as assistant to the president for intergovernmental affairs, according to Mrs. Reagan's office.

In accepting Rosebush's resignation, Mrs. Reagan said in a statement: "Jim and I have traveled many miles together and we have become good friends. I sincerely appreciate all he has contributed to the drug program and many other aspects of the first lady's office. I will miss him."

Verstandig left the White House for HUD last May. Prior to that he served as acting administrator of the Environmental Protection Agency and assistant secretary of transportation for governmental affairs.

Verstandig said in an interview yesterday that his salary will be "very comparable" to Rosebush's. But unlike Rosebush, who is a deputy assistant to the president, Verstandig will join Mrs. Reagan's staff as an assistant to the president -- the same rank he held when he left.

"Maybe that adds some stature in terms of relations with the West Wing," Verstandig said.

Rosebush, 36, who joined the first lady's staff in February 1982, was rumored to have been unhappy for some time over his failure to attain the rank of assistant. Those rumors intensified after Donald Regan took over as White House chief of staff last winter and brought in Dennis Thomas as an assistant, reportedly to oversee Mrs. Reagan's East Wing operation. Rosebush said in an interview in early October, however, that nobody ever told him that Thomas was his supervisor.

"I knew he was being brought in as Regan's assistant," Rosebush said. "More than that, frankly, it's never been spelled out to me."

Rosebush was a prote'ge' of former deputy White House chief of staff Michael Deaver, who left last spring to open his own public relations firm. In an October interview, Rosebush said liaison with the president's side of the White House was "not what it was" when Deaver was in charge.

"I go to senior staff meetings in the morning at Don Regan's. I'm involved in scheduling meetings, but things have changed. I wouldn't say it isn't different," Rosebush said then.

Rosebush said yesterday he isn't leaving because of any problems on the job. He said he "couldn't be happier. I'm very happy in my job." When he told Mrs. Reagan 2 1/2 weeks ago that he intended to leave, it came as "a big surprise" to her.

Rosebush said he made his decision about three weeks ago when he "bolted straight up out of bed one morning and said, 'It's going to be four years and I think it's time to leave.' " Since then, he said, he has been "considering various options," but has made no decisions.

"I thought this was really the best way to do this," Rosebush said. "I'm concerned with a smooth transition for Mrs. Reagan; I've been concentrating on her and making the transition right for her. We're announcing it now because Lee will be going with us to [next week's Reagan-Gorbachev summit in] Geneva and there would be questions."

A native of Flint, Mich., Rosebush came to Washington from Standard Oil of Ohio, where he was in charge of the company's $10 million corporate contributions program. He worked at the Department of Commerce, where he headed the office of business liaison, before moving to the White House, where he started the Private Sector Initiatives office in 1981.

As Mrs. Reagan's chief of staff, he coordinated a staff that succeeded in turning around public perceptions of the first lady, from those of a woman with frivolous interests to someone determined to overcome drug abuse among American youth.

Rosebush and his wife Nancy have two children, whom he described as "another consideration" in his decision to leave the White House. "I'd like to spend more time with my family," he said.

Verstandig, a native of Memphis, has a PhD from Brown University, where he later became associate dean of academic affairs. A longtime Republican Party volunteer, he worked in the Nixon and Goldwater presidential campaigns. He also worked in former Rhode Island governor John Chafee's 1976 campaign for the U.S. Senate, and after Chafee won came to Washington as his chief of staff.

In the 1980 presidential campaign, Verstandig headed the Reagan "Truth Squad" of senators and congressmen who campaigned around the country. In 1981, he went to work for then-Secretary of Transportation Drew Lewis, mapping his legislative strategy. He became a troubleshooter for President Reagan, who sent him over to the EPA in February 1983.

"The president asked me to go over and help Anne Gorsuch at EPA, and after she resigned they made me the acting administrator. So I was the poor person who had to reorganize that place, fire some people and help get Bill Ruckelshaus confirmed," Verstandig said.

In his interview with Nancy Reagan on Nov. 4, he said he told her that he had been involved in drug issues at Brown, helping put together a drug information center for undergraduates. He also served on the board of Marathon House, a drug rehabilitation center.

Verstandig is married to Toni Grant Verstandig, whom he describes as "a conservative Democrat," who is a staff member of the House Foreign Affairs Committee. Verstandig is a nephew by marriage to the founder of Madame Wellington, czarina of paste jewelry. He has a son from a previous marriage.