Buzz Aldrin, the second man to walk on the moon, was evacuated by plane from the South Pole after falling ill.
The eighty-six-year old was visiting Antarctica as part of a White Desert private tourist group when his health deteriorated, the International Association of Antarctica Tour Operators said in a statement.
Mr Aldrin was flown to McMurdo Station, a US research centre on the Antarctic coast, on a cargo plane on the recommendation of doctors from White Desert and the US Antarctic Program (USAP).
From there he was taken to Christchurch, New Zealand where he is recovering in hospital.
Mr Aldrin had fluid on his lungs, but was in good spirits and responding well to treatment, said White Desert in a statement.
The former astronaut had earlier tweeted about his trip to Antarctic writing: "South Pole here I come" accompanied by a picture of him alongside a plane.
In another Twitter post he posed with other members of the tourist group and wrote: "Buzz's Polar Penguins countdown to liftoff has commenced".
The National Science Foundation described Mr Aldrin as "ailing", but his condition was thought to be stable.
Mr Aldrin became the second man to walk on the moon, about 20 minutes after Neil Armstrong made his famous first steps in 1969 as part of the US Apollo 11 mission.