Both are members of the House of Representatives Committee on Foreign Relations and leading members of the Cuba Working Group, which aims to foster better political, economic and cultural ties.
Fidel Castro has not been seen in public since July 26, the day before his surgery, although he has appeared in videotaped visits in his hospital room.
Fidel, 80, temporarily stepped aside on July 31, handing power to his brother and defence minister Raul, 75.
The unprecedented scope of the personalised push by Republican and Democratic legislators came two weeks after Raul signalled openness to the possibility of dialogue with the US.
The American delegation met on Friday with Ricardo Alarcon, the president of Cuba's National Assembly and the top official for US affairs.
Among Saturday's round of events was a reception with Felipe Perez Roque, the foreign minister, and meetings with Francisco Soberon, the central bank governor, and Yadira Garcia, the basic industries minister, an influential member of the politburo of Cuba's Communist Party.
Authorities did not rule out a meeting with Raul or Carlos Lage, the vice-president, organisers of the visit said privately.
The US legislators have been tightlipped during the visit but have scheduled a news conference on Sunday.
The timing for the unprecedented visit came hours after John Negroponte, the US director of national intelligence, said that Fidel Castro was close to death.
Negroponte said: "Everything we see indicates it will not be much longer ... months, not years."
The US politicians' arrival in Cuba came on the day that Hugo Chavez, the Venezuelan president, denied reports that Fidel has cancer, saying that he was instead fighting a "great battle" against a "serious" illness.
However, the US has indicated no interest in talks without signs of political liberalisation in Cuba.
On Wednesday, Tom Shannon, the most senior US diplomat for Latin America, said that Washington had yet to see any reformers in the Cuban government.
"When we engage it has to be part of a democratic change."
Tom Shannon, US diplomat in Latin America
He said: "Once [Fidel Castro] goes, the successor government is going to have to chart out some kind of path into the future. There are no clear signals of what that path is going to be.
"We don't see any significant possibility of change of any kind until Fidel is gone."
Shannon said that any US engagement with Cuba must be "part of a change process that facilitates a democratic transition".
"We are attentive to what is happening in Cuba, to what would happen after Fidel Castro passes from the scene. When we engage it has to be part of a democratic change."