CAPE CANAVERAL, Fla. -- The International Space Station is experiencing a slow, steady drop in air pressure, and American and Russian flight controllers are investigating possible causes of the leak.

Mission Control notified astronaut Michael Foale and cosmonaut Alexander Kaleri about the leak just before their bedtime late Monday afternoon.

"There's no action for you at this time and no immediate concerns," Mission Control assured the two men. "We'll continue to investigate this on the next shift and we may have some actions for you tomorrow."

Not long afterward, Foale radioed back that the men had gone ahead and done some checks of their own. He reminded Mission Control that Kaleri, one of the last two men on Mir, was sent to the Russian outpost in 2000 to look for a similar leak and was doing a thorough check of all the accessible valves leading to the vacuum of space.

The valves on the U.S. side of the space station checked out fine, Foale said. The men then turned their attention to the valves inside the Russian compartments and also the Russian cargo ship docked to the complex.

They found nothing amiss.

"We're going to call it a night and make sure we don't misplace the leak (detection) equipment," Foale said with a chuckle.

Mission Control first noticed the drop in pressure Jan. 1 and said the data showed a daily decline of about 2 millimeters of mercury. As of Monday, the pressure had declined a total of nine millimeters. That is equivalent to about one-quarter of a pound per square inch, said NASA spokesman James Hartsfield.

Normal pressure inside the space station is 14.7 pounds per square inch, the same as at sea level.

Foale knows firsthand about falling pressure in space. He was aboard Russia's Mir space station in 1997 when a supply ship rammed into a lab module and ruptured it; the lab was quickly sealed off from the rest of the outpost.

Back in November, Foale and Kaleri reported a loud noise that sounded like a flapping sheet of metal. The air pressure remained stable, however, and all the other systems seemed fine, too. NASA has yet to determine what caused the noise.

NASA also is monitoring a malfunctioning gyroscope used for motion control and a broken oxygen generator. Spare parts are at a premium because of the nearly yearlong grounding of the space shuttle fleet, the result of the Columbia tragedy.

Foale and Kaleri are 21/2 months into a six-month space station stay. They rocketed into orbit from Kazakhstan in October and are to return there via a Russian capsule in April.

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