21 December 2013
Last updated at 14:02 ET
It is the first of three possible spacewalks needed to mend the station's critical cooling system.
Half of the system automatically shut down last week after detecting abnormal temperatures.
Nasa said the situation was potentially serious but not life-threatening.
The six-man crew had to turn off all non-essential equipment because of the malfunction.
Faulty pump The two astronauts, Rick Mastracchio and Mike Hopkins, began their spacewalk - 418km (260 miles) above the Earth's surface - at about 1200 GMT and were outside the space station for nearly six hours.
They disconnected and removed a faulty ammonia pump, which is about the size of a refrigerator.
"I got it, I got it, I got it. Barely," Rick Mastracchio exclaimed as he stretched out his hand to retrieve an o-ring which began to float away.
The spacewalk was curtailed slightly as he complained of excessive cold in his feet.
The astronauts wore absorbent equipment in their space suits to prevent any repeat of the helmet-flooding problem which affected an Italian astronaut earlier this year.
On their second spacewalk, planned for next Monday, the two astronauts will install a new pump.
It is possible that a third expedition will be needed, on Christmas Day, to make final installations. But Nasa said this may not happen because of the success of Saturday's operation.
The fault relates to the external cooling loops that circulate ammonia outside the station to keep both internal and external equipment cool.
Nasa said the repairs would take priority over the launch of a supply ship from Virginia, which has now been postponed until January.
Nasa astronauts carry out spacewalk to repair pump
The two American astronauts on the first of a series of urgent repair spacewalks
Astronauts at the International Space Station have completed the first of a series of spacewalks to carry out urgent repairs.
The two Americans on the crew went outside the station and removed a pump containing a faulty valve.It is the first of three possible spacewalks needed to mend the station's critical cooling system.
Half of the system automatically shut down last week after detecting abnormal temperatures.
Nasa said the situation was potentially serious but not life-threatening.
The six-man crew had to turn off all non-essential equipment because of the malfunction.
Faulty pump The two astronauts, Rick Mastracchio and Mike Hopkins, began their spacewalk - 418km (260 miles) above the Earth's surface - at about 1200 GMT and were outside the space station for nearly six hours.
They disconnected and removed a faulty ammonia pump, which is about the size of a refrigerator.
"I got it, I got it, I got it. Barely," Rick Mastracchio exclaimed as he stretched out his hand to retrieve an o-ring which began to float away.
The spacewalk was curtailed slightly as he complained of excessive cold in his feet.
The astronauts wore absorbent equipment in their space suits to prevent any repeat of the helmet-flooding problem which affected an Italian astronaut earlier this year.
On their second spacewalk, planned for next Monday, the two astronauts will install a new pump.
It is possible that a third expedition will be needed, on Christmas Day, to make final installations. But Nasa said this may not happen because of the success of Saturday's operation.
The fault relates to the external cooling loops that circulate ammonia outside the station to keep both internal and external equipment cool.
Nasa said the repairs would take priority over the launch of a supply ship from Virginia, which has now been postponed until January.
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