The Cuban leader, Fidel Castro, has temporarily handed over power to his younger brother while he recovers from intestinal surgery.
His 75-year-old brother, Raul Castro, is currently defence minister.
In a statement, President Castro said he had over-exerted himself during a visit to Argentina, and last week's anniversary of the Cuban revolution.
It is expected he will have to rest for several weeks.
Fidel Castro, who will be 80 this month, has led Cuba since he seized power from dictator Fulgencio Batista in January 1959.
US President George W Bush, has maintained a tough policy toward Cuba and a recent US government report recommended that the US should act fast to shape a transitional government in Cuba when Castro's rule ends.
The two countries have no diplomatic ties and the US has maintained an economic embargo on the island for more than four decades.
