Monaco ? Prince Rainier III of Monaco, who was hospitalized two weeks ago with a chest infection, has been placed in the intensive care unit, the palace said Tuesday.

After a marked improvement, “a recurring pulmonary infection required that the prince be transferred into an intensive care unit,” the palace said in a statement.

The 81-year-old Rainier, whose movie-star wife, Grace Kelly, died in a car crash in 1982, has a history of heart problems and has suffered from poor health in recent years.

Rainier was admitted March 7 to a heart and chest clinic due to a “bronchial-pulmonary infection.” The palace said three days later the initial infection had been “quickly brought under control.” Rainier’s doctors then decided he should undergo a series of cardiac examinations, given his past history of heart problems.

Rainier spent a week at the same Monaco hospital in October for treatment of a chest infection. He also was admitted to the clinic in February 2004 for a coronary lesion and a damaged blood vessel, and spent three weeks there the month before for what was described as general fatigue.

Monaco, a tiny principality famous for its casinos, Formula One Grand Prix and tax breaks that attract the rich and famous, is nestled on the Mediterranean Coast between Italy and the French Riviera.