Nelson Mandela, the former South African president, is suffering from a recurring lung infection but is responding to treatment, officials said on Tuesday.
The 94-year-old became ill on Saturday and was taken to the 1 Military Hospital near the capital, Pretoria, reports the Associated Press.
In an interview with ENews Central Africa (ENCA) on Monday, Mandela's wife Graca Machel, described how Mandela's "sparkle" was fading.
She added: "To see him ageing, it's something also which pains you. You understand and you know it has to happen."
A presidential spokesman Mac Maharaj said the 94-year-old is "receiving appropriate treatment and he is responding to the treatment."
According to the Daily Mail, Mandela had not been talking during his stay although a spokesman insisted there was "no cause for alarm.
South Africa's first black president and a Nobel Peace Prize winner, Mandela has previously been treated in hospital for a serious chest infection in 2011, and also for an abdominal problem earlier this year.
Officials have refused to speculate as to when he might be discharged.