Venezuela sells Cuba about 90,000 barrels of crude oil per day on preferential terms, and hopes to increase shipments to 150,000 barrels per day by 2013.
Venezuela's state-run oil company has also invested some $83m since 2006 to rehabilitate Cuba's Soviet-era Cienfuegos refinery, Rafael Ramirez, the Venezuelan oil minister, said in July.
"Venezuela is Cuba's most important commercial partner ... now that the prices of oil has slumped President Raul Castro needs to know that that oil will continue to be flown into Cuba - that that money will continue to come in," Mariana Sanchez, Al Jazeera's correspondent in Venezuela, said.
"By some estimates these subsidies are even higher than the subsidies that the Soviet Union used to send Cuba during Soviet times."
Castro, wearing sunglasses and a gray suit, delivered greetings from his older brother.
"I bring a salute, a hug for all Venezuelans from the Cuban people and from the leader of the revolution, comrade Fidel Castro," he said.
Chavez and Raul Castro laid flowers at a statue of Simon Bolivar, the South American independence hero, in the capital's centre.
It was not clear exactly how long Castro planned to stay in Venezuela, but Venezuela's official news agency said the visit was going to last just a few hours.
Castro was expected to head next to a regional integration summit in Brazil, which opens on Tuesday.
Since taking office, Raul Castro has bolstered ties with Russia and China and is now seeking to take advantage of Brazil's stated goal of boosting Cuban trade, especially in its nascent oil industry.