In a rare scare, astronauts fled the American side of the International Space Station after an alarm indicated a possible toxic leak.
Nasa later said a computer problem probably set off the alarm rather than escaping ammonia coolant.
"No signs of a leak," Nasa reported via Twitter.
In the meantime, the six crew members huddled safely on the Russian side of the orbiting outpost as Mission Control analysed the data.
The astronauts had to rush over to the Russian segment twice: the first time when the alarm sounded, the second following an initial all-clear.
"Hey everybody, thanks for your concern," Italian astronaut Samantha Cristoforetti said in a tweet. "We're all safe & doing well in the Russian segment."
The "unscheduled excitement", as Nasa called it, occurred at around 9am GMT, well into the station crew's workday.
As alarms blared, the astronauts followed emergency procedures in slapping on oxygen masks, taking cover in the Russian quarters, then sealing the hatches between the US and Russian sides.
At the same time, flight controllers at Nasa's Johnson Space Centre in Houston turned off non-essential equipment.
Within minutes, Mission Control gave an all-clear but sent the astronauts scurrying back over to the Russian side again when there was more evidence of a possible leak.
The highly toxic liquid ammonia, flowing outside the space station, is used to cool electronics. Flight controllers originally feared it had got into the water system running inside. Now it is believed that a failed card in a computer-relay box was the culprit.
The crew - three Russians, two Americans and the Italian Cristoforetti - will remain in the three, relatively small Russian compartments in the meantime.
Space station programme manager Mike Suffredini said while it is inconvenient for all six to be isolated on the Russian side, there are no health or other concerns. Controllers hoped to give the final all-clear by Wednesday night.
Engineers want to understand the computer failure and confirm "that the system is tight like we believe it to be", Mr Suffredini said on Nasa TV. He added: "We'll do due diligence to make sure we're comfortable with the environment before we send them in there."
Russian space officials at first reported an actual leak, then backed off that statement.
Commander Butch Wilmore, an American, and his crew were dealing with supplies and experiments from the newly arrived SpaceX capsule - including fruit flies - when the ammonia-system alarm sounded. None of the research appears to be jeopardised, Mr Suffredini said.
The 260-mile-high complex has never had to be abandoned during its 14-year-plus occupation by astronauts.
On occasion, crews have had to seek shelter in their Soyuz capsule "lifeboats" because of close shaves with orbiting junk, in case a quick getaway was needed.
This time, the astronauts went into one of the three Russian modules as trained for an emergency, but not the two docked Soyuz capsules.
Besides commander Wilmore and Italian Cristoforetti, the crew includes American Terry Virts and Russians Elena Serova, Alexander Samokutyaev and Anton Shkaplerov.