Pope John Paul has cancelled his weekly general audience for later this morning because of an intestinal problem, Vatican sources say.
The two sources said the problem was not serious but the pope's doctors had advised him to rest.
The pope has cancelled his general audiences on short notice only a handful of times during his papacy of nearly 25 years. The 83-year-old pope was to have returned to the Vatican from the papal summer residence at Castel Gandolfo, south of Rome, to address thousands of people at the Vatican.
One source said the pope still planned to move back to the Vatican from the summer residence on Friday afternoon and, health permitting, preside at a Vatican ceremony on Saturday.
While the current intestinal problem was deemed not serious, the pope's doctors want him to be careful because there could be complications with his other ailments.
The pope, who suffers from Parkinson's disease and no longer walks, was weak during his trip to Slovakia earlier this month.
On that trip, he was unable to pronounce his words clearly and aides had to read most of his sermons and addresses for him.
The pope will need all the strength he can muster for activities next month marking the 25th anniversary of his election on October 16th, when the world will be watching.
On October 19th, he is due to beatify Mother Teresa of Calcutta.