The space agency is seeking proposals for the third and fourth private astronaut trips to the orbiting lab.
According to a partnership between NASA and the private company Axiom Space, the first all-private crew was sent to the ISS in April.
For the second private flight to the ISS, NASA chose Axiom Space.
According to NASA, the two next space station missions might spend up to 14 days in orbit.
The objective of the space agency is to create a "strong low-Earth orbit economy" through these individual journeys to the ISS.
NASA selected Axiom Space to conduct the second private mission to the ISS.