The astronauts -- Chris Cassidy and Tom Marshburn -- have removed "a 260-pound pump controller box that may be the source of an ammonia leak on the International Space Station and replaced it with a spare", NASA said in a statement late evening.
The emergency spacewalk to "inspect and possibly replace a pump controller box suspected of leaking ammonia coolant" began at 1244 GMT Saturday after a team of NASA officials gave the go-ahead late Friday.
The leak was reported to mission control in Houston Thursday, when the ISS crew said they could see "a very steady stream" of small white flakes floating away from an area of the truss structure.
The astronauts used hand-held cameras and, with help from mission control, were able to narrow down the location of the leak to the truss. They also determined that the rate at which ammonia was seeping out of the cooling loop was increasing.
Chilled liquid ammonia is used to cool the power channels on the ISS' eight solar array panels, which supply electricity to the ISS. Each solar array has its own cooling loop.
The ammonia loop where the leak was detected is the same one that spacewalkers tried to fix a leak on last November, NASA said.
In Moscow, an official from the Russian space agency said the leak had occurred in the US portion of the vessel.