The voice of the phone-in caller on the Venezuelan radio talk show sounded familiar but it took the programme's host, president Hugo Chávez, a moment or two to recognise it. "My God," he said. "It's Fidel!"
In a half-hour call to Mr Chávez's weekly radio show, Castro said that he was feeling much better and complained at the constant pressure to provide regular updates on his health.
"How are you?" asked Mr Chávez in English. "Very well," Castro replied, also in English, to the amusement of his main ally in Latin America.
"I'm gaining ground," said the 80-year-old Cuban president, who is recovering from a complicated abdominal operation. "I feel more energetic, stronger and I have more time to study. I've become a student again."
During the programme, which was later broadcast on Cuban national television, Mr Castro said: "I can't promise that I'll get over there soon but, yes, I'm gaining ground. I feel good and I am happy."
President Chávez, who has visited the ailing leader in Cuba on two occasions and has given regular updates on his health, told him: "You don't know how happy we are to hear your voice and know that you're well." He referred to his caller, who transferred power to his brother Raúl last year, as "brother" and "comrade Fidel".
The pair also went on to discuss this week's plunge in US and Chinese stocks, which they agreed should concern the US government, the sworn enemy of both men.
Mr Castro also complained of "the habit, the vice" of people's expectations of constant updates on his condition. Referring to his legendary ability to talk for hours, he acknowledged that that part of his life, at least, had changed.
"I can't talk every day," he said in a soft voice. "I ask everyone for patience, calm ... the country is marching along, which is what is important ... I ask for peace also for me so that I can fulfil my new tasks."
Mr Chávez told him that he would win "the battle for life". The two men ended the conversation with their familiar cry of "fatherland or death!"