A medical update from the palace described the 81-year-old prince's condition as stable a day after he was moved into intensive care at Monaco's Cardio-Thoracic Center.
The prince, whose actress wife, Grace Kelly, died in a 1982 car crash, was hospitalized more than two weeks ago with a chest infection. After a marked improvement, his health suddenly worsened.
A palace statement Wednesday said Rainier was transferred to the intensive care unit after developing a sudden respiratory infection "with cardiac and kidney failure."
"Breathing difficulties made the installation of artificial respiration indispensable," the statement said.
Outside the hospital, life continued as normal in this tiny principality wedged between the mountains and the Mediterranean. Some residents watched the palace — and its flagpole — for signs.
"While the flag is still up, we know he's still alive and all is well," said Sandrine Negre, 22, out strolling with friends near Rainier's seaside hospital. "All Monegasques are watching that flag."
Rainier, who assumed the throne in 1949, is revered by his subjects for having transformed Monaco — which is smaller than New York City's Central Park — into a modern and elegant resort that is a magnet for jetsetters.
"This country is Prince Rainier," said Patricia Vermeulen, a 53-year-old retired teacher who lives near the palace. "This fabulous adventure that is Monaco, he created it."
Speaking his name brought tears to her eyes.
"We've known about his bad health for a long time. But each new time I feel the deepest sadness, as if it were my father," said Vermeulen, setting down grocery bags to dab her eyes. "It's like a knife in my heart each time."
"He's not gone," she paused. "Not yet."
Crown Prince Albert, 47 — Rainier's heir — returned from abroad Tuesday to visit his father, as did Princess Caroline, 48. Princess Stephanie, 40, was seen entering and leaving the hospital.
In 2002, the constitution was revised to allow Albert to succeed his father, despite his lack of heirs. According to Article 10 of the constitution, Princess Caroline would succeed Albert should he die without children. She, in turn, would be succeeded by her oldest son, Andrea Albert Pierre, now 20.
Archbishop Bernard Barsi of Monaco visited the hospital Tuesday evening. On Wednesday, the Rev. Philippe Blanc was seen entering the clinic.
The visits were a somber note to what has been a high-flying lifestyle for the Grimaldi family and their playground for the rich that has become synonymous with casinos, Formula One races and tax breaks.
Rainier's two daughters for years have been the focus of paparazzi, who fed off their rocky love lives. Stephanie, known as the wild child, had three children out of wedlock, then married a circus acrobat.
Rainier has a history of heart problems and has recently been plagued with recurring ailments linked to his respiratory tract.
Infections can bring on congestive heart failure, which can lower blood pressure and ultimately lead to kidney failure. Heart failure also depresses the respiratory system, making breathing difficult.
Doctors often use respirators and dialysis machines to lighten the workload of the body while healing from an infection. Once the infection is cleared, the machines can be disconnected and the body can resume its normal function, said Dr. James Underberg of the New York University Medical Center.
However, respiratory infections in the elderly can be dangerous and deadly, Underberg noted.
"It could go either way. This could be a temporary thing where they just support his body while they aggressively treat the infection with antibiotics and he recovers or it could be the beginning of a downward spiral," he said.
Rainier spent a week at the same Monaco hospital in October because 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.