Surprise call
His guest appearance came as a surprise to listeners. Chavez said his energy minister was on the line but the 80-year-old Cuban's voice broke in: "Listen, my distinguished and dear friend, how are you?"
"Goodness, it's Fidel," Chavez replied.
Then the two icons of Latin America's left, who pride themselves on their anti-Americanism, exchanged pleasantries - in English.
"Fidel, how are you?" Chavez said with a heavy accent.
"Very well," Castro replied as they both laughed.
Castro later commented on the fall of world stock markets on Tuesday, the worst in years, and said it was proof of his view that capitalism was in crisis.
Castro and Chavez touched on political and economic topics such as producing ethanol as an energy source and Venezuela's industrial output.
Chavez ended the call, saying, "I declare to the world that you are my father". Castro signed out with the revolutionary slogan, "We will overcome".
In power since 1959, Castro handed over control temporarily to his brother, Raul, on July 31.
Castro's condition is a tightly guarded state secret. His illness was officially attributed to the "extreme stress" of his workload.