The alarm sounded with about five minutes remaining in the countdown. NASA counted down to the 31-second mark and debated what might have caused the master alarm to go off. Managers counted down several more seconds before calling off the attempt for the day.

Until practically the last minute, NASA worried that rain and accompanying clouds would foil the launch. But the rain drifted away in time for the targeted 3:58 am liftoff -- and then the alarm went off.

Endeavour pilots reported that the alarm sounded right around the shuttle hydraulic power units were turned on. But launch controllers quickly determined that all three units were working fine.

NASA spokesman Bruce Buckingham said a second launch attempt could be made tomorrow, but that any such decision would have to bemade by the engineers.

Secretary of State Madeleine Albright and other dignitaries from around the world had gathered in the drizzle to see Endeavour and its crew of six off following a year's delay caused by a cash crunch in Russia. Most were representatives of the 16 countries participating in the space station effort.

``This is a visionary idea,'' Albright told mediapersons. ``This is an investment in the future.'' The mission promises to be one of the most difficult in space shuttle history. And the entire space station project promises to be the most difficult undertaking since the Apollo moon landings nearly 30 years ago.

``We're not testing just hardware as part of this program,'' said Mikhail Sinelschikov, a Russian space official. ``We're testing our relationship very much and our mutual trust and our confidence in each other.''

Two weeks ago, the Russians put the first piece of the space station into orbit. During Endeavour's 12-day flight, the astronauts will use the shuttlerobot arm to capture the Russian piece and attach it to the American component. Then, two astronauts will perform three spacewalks to hook up all the electrical connections and cables between the two cylinders.

The US piece, named Unity, is a connecting passageway. It is essentially an aluminum can with big holes where other chambers eventually will attach. It contains 10 km of wiring and more than 50,000 mechanical pieces.

The Russian component, named Zarya, which means sunrise, contains the power and propulsion systems that will keep the station aloft until the Russians can launch a permanent control module.

In all, more than 100 components weighing one million pounds (450,000 kg) are to be launched over the next five years, requiring 45 US and Russian flights.

``It's going to start out kind of small, but it's going to grow to the size of a football field,'' said shuttle commander Cabana.

Unity should have been put in orbit last December. But station assembly was put on hold because of Russia'seconomic crisis, which prevented the country's space agency from completing the permanent control module. NASA has since bailed out the Russian space agency.