The International Space Station is experiencing a loss of pressure that could be the result of a leak.
Construction work on the 16-nation space platform is on hold
The US space agency Nasa says the two men on board are in no danger and have been informed of the situation.
US mission commander Michael Foale and Russian colleague Alexander Kaleri are spending six months on the platform.
Space officials say they spent some time on Monday inspecting valves and windows on the station but found "nothing unusual" and went to sleep.
The crew were told of the problem by flight controllers. Nasa said in a statement: "The decay, which is currently measured at a rate of less than 0.04 pounds per square inch a day, is having no impact on station operations and the crew is in no danger."
British-born Foale and his Russian colleague thought they had been hit by an object that might have caused a leak last November.
They heard a metallic crushing sound as they were eating breakfast but it turned out to be a false alarm.
Russian controllers said there appeared to be no damage to the outside of the platform or change in air pressure inside, and later confirmed the noise came from an onboard instrument.