The 81-year-old Rainier was hospitalized two weeks ago with a chest infection. After a marked earlier improvement, a recurring lung infection called for more specialized care, a palace statement said. It did not say when Rainier was transferred to intensive care.
Rainier, whose wife, Princess Grace, died in a car crash in 1982, has a history of heart problems and has been in poor health in recent years. He has been hospitalized repeatedly in the past decade.
The couple's three grown children, Crown Prince Albert, Princess Caroline and Princess Stephanie, visited their father in the hospital, an official said, speaking on condition of anonymity. Both Albert, 47, and Caroline, 48, had been abroad. Stephanie, 40, was seen entering and leaving the hospital.
On March 7, Rainier was admitted to the Monaco Cardio-Thoracic Center for what was described as a bronchial-pulmonary infection.
The palace said March 10 that the initial infection had been "quickly brought under control." Rainier's doctors then decided he should undergo a series of cardiac tests, given his history of heart problems.
Doctors have not detailed the nature of Rainier's ailment, but lung infections, such as pneumonia, are common - and often life-threatening - in older people.
"Someone in their 80s could develop multiple types of infections which are life threatening," said Dr. Michael Freedman, a gerontologist and professor at New York University School of Medicine. "Pneumonia puts a strain on the heart and often causes congestive heart failure."
Patients with chest infections are usually put into intensive care for two reasons: the infection is not under control and is causing breathing difficulties that require a breathing tube in the throat, or because doctors want to check for heart failure, Freedman said.
Rainier spent a week at the same hospital in October for 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.
Rainier assumed the throne in 1949. His heir is Albert, who is unmarried and has no children. Monaco changed its succession law in 2002 to allow power to pass from a reigning prince who has no descendants to his siblings. Both of Albert's sisters have children.
Monaco, famed 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.
By Thierry Boinet and Emma Ross