Former South African president Nelson Mandela was in a "serious but stable" condition after being taken to hospital early on Saturday with a recurring lung infection, the government said in a statement.
The 94-year-old anti-apartheid hero and the country's first black president has been in hospital three times since December and has been battling this bout of the infection for several days, it said.
"This morning at about 1:30 a.m. (2330 GMT) his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition," the government said.
Presidential spokesman Mac Maharaj told local television Mandela's condition was "serious this time".
"The situation is serious this time but doctors have assured us he is comfortable," he told television station eNCA.
The Nobel Peace Prize laureate stepped down as president in 1999 and has not been politically active for about a decade.