"This is a significant time for us. We've been preparing for four months for this in many ways," said Culbertson, a former Navy test pilot fond of nautical traditions.

"We have been taking care of the station. We've been remodeling. We've been adding rooms. We've been doing many things to ensure the station is ready for the next crew while conducting our business, conducting our research, and doing our assigned tasks," he said.

"Expedition 4, the ship is now your responsibility."

Noting that their time on the station had flown by, Turin wished the new crew "as much success as possible" and urged them to be patient "because they will have some troubles, they will have some difficulties."

Dezhurov, the pilot responsible for maintaining a Soyuz spacecraft parked at the outpost, added the emergency lifeboat would be ready for any trouble that might force the station's new tenants to abandon ship.

"I can say, Yuri and you guys, this is your spacecraft (now), and this is a very good spacecraft. And the spacecraft is completely ready for a return to Earth if there is some difficult situation."

Ferried up to the station aboard shuttle Endeavour, Onufrienko, Bursch and Walz actually took up residence on the outpost last Saturday after custom-made seat liners were installed in the Soyuz lifeboat.

The trio since then has been setting up house in station staterooms while Culbertson, Dezhurov and Turin have been staying aboard the shuttle.

The change-of-command ceremony simply formalized the handover while marking the end of a station expedition that was geared more toward scientific research than outpost construction.

The Expedition Three crew oversaw the delivery and installation of a new Russian airlock in September and carried out three spacewalks to outfit it externally. But most of the crew's time was spent conducting more than 50 U.S. and Russian science experiments.

Covering a wide range of scientific disciplines, the experiments included medical studies aimed at gauging the effect of spaceflight on the human body, research deemed key to preparing for future missions to the moon, Mars or beyond.

"I'm very proud of what we've accomplished," Culbertson told reporters in a space-to-ground news conference earlier this week.

"We accomplished all the tasks that were given to us, the big ones and the little ones. We probably did even a little bit more than what people expected of us in some areas. So I'm very happy with what we achieved. I'm very happy with the state of the station at this point, and I'm very proud to be turning it over to Expedition Four."

During what promises to be the longest stay at the station to date, the Expedition Four crew plans to carry out 65 U.S. and Russian science experiments in areas that include medicine, physics and pharmaceutical research.

Senior NASA station scientist John Uri calls it "the busiest, most diverse and most complex research program we've ever attempted on the International Space Station."

What's more, Onufrienko and his crew will oversee the delivery and installation of the central segment of the station's metallic backbone, a girder-like truss that eventually will stretch 356 feet (108 meters) from tip to tip.

The three veteran space fliers also will conduct or support as many as eight spacewalks, six of which might be needed to install the truss segment and prepare the station for the arrival of two more metal girders in late 2002.

The other two sorties primarily will involve repositioning a Russian construction crane, installing protective deflector shields around station jet thrusters and setting up ham radio antennas outside the outpost's Russian-built crew quarters.

Onufrienko, Bursch and Walz also will play host next April to a visiting Soyuz taxi crew that will include Internet tycoon Mark Shuttleworth, 28, who is destined to become the first South African to fly in space.

The Endeavour astronauts and their station colleagues, meanwhile, pressed ahead Thursday with the repacking of an Italian moving van that was hauled up to the outpost on the shuttle.

More than two tons of luggage, surplus station gear, garbage and scientific experiment samples are being loaded aboard the cylindrical cargo carrier for the shuttle's scheduled return to Earth next week.

Earlier this week, the astronauts and cosmonauts unloaded 3.5 tons of food, clothing, supplies and equipment from the pressurized freighter for Onufrienko and his crew. The moving van -- which was temporarily mounted to the station Monday with the shuttle's robot arm -- will be stowed back in Endeavour's cargo bay Friday afternoon.

With the repacking work nearly done, the visiting shuttle astronauts and the two station crews also forged ahead with some maintenance work at the outpost.

They swapped out six utility outlets but then ran into some trouble testing a high-pressure system designed to transfer oxygen from the shuttle to storage tanks on the station's U.S. Quest airlock. The trouble appeared to be with an oxygen hose fitting, and the test was postponed so that ground engineers could study the problem.

The shuttle astronauts and the Expedition Three crew remain scheduled to depart the station Saturday, a day later than originally planned. The shuttle's stay at the station was extended so that Endeavour's astronauts could help out with maintenance work on the outpost.

A farewell and hatch closing ceremony is set to take place about 8:30 a.m. EST (1330 GMT) Saturday, and Endeavour is scheduled to pull out of the station two-and-a-half hours later.

Launched last Wednesday from Kennedy Space Center, the shuttle is scheduled to land at NASA's coastal Florida spaceport at 12:55 p.m. EST (1755 GMT) Monday.