In a brief written statement, the Vatican said the 84-year-old pontiff, whose tenuous health took a sharp turn for the worse on Thursday, was still responding when addressed by aides.
"The clinical conditions of the Holy Father remain very serious. In the late morning a high fever developed. When addressed by members of his household he responds correctly," the statement said.
Earlier on Saturday, the Vatican said the pope had begun to slip in and out of consciousness but was not in a coma.
Italian news agency ANSA quoted unidentified health sources as saying the Polish pope was progressively losing consciousness and his situation was irreversible.
The pope, who suffers from Parkinson's disease, failed to recover from recent throat surgery aimed at helping him breathe.
On Friday, the Vatican said John Paul had suffered a urinary infection that triggered septic shock - a life-threatening assault by bacteria on the bloodstream. His heart and kidneys
then began failing and his blood pressure fell dangerously low.
Tourists and pilgrims streamed anew into St Peter's Square on Saturday, and around the world, priests readied Roman Catholics for the Pope's passing. Many expressed hope that his final hours would be peaceful.
In Wadowice, Poland, where the Pope was born, people left school and work early and headed to church to pray for their native son.
Hospitalised twice last month after breathing crises, and fitted with a breathing tube and a feeding tube, John Paul has become a picture of suffering.