"Glad you left the lights on for us," astronaut James Voss told Mission Control.
Astronaut Susan Helms and Russian cosmonaut Yuri Usachev opened the hatch to the brightly lit US module Unity at 22:03 GMT on Monday and entered the Russian module Zarya, or Sunrise, some 55 minutes later.
"Glad you left the lights on for us," astronaut James Voss told Mission Control.
The visit to the space station modules was a sneak preview for Usachev, Helms and Voss, who will spend five months in the orbiting platform next year after the first full-time crew have left.
The air tested fine, although the temperature was around 30 degrees Celsius, prompting one astronaut to strip down to his socks and a pair of shorts.
Personal fans
They quickly began removing the first of four 73-kilo (163-lb) batteries, which have been failing due to careless overcharging.
Each astronaut had small, personal fans to prevent exhaled carbon dioxide from pooling around their heads.
The last time astronauts visited the space station, they suffered nausea, headaches and eye irritation, thought to be because of the stagnant air.
Before Atlantis undocks on Friday, the astronauts will also boost the sagging space station in orbit and bring over half a tonne of supplies for use by future crews, including clothing, computers and exercise equipment.
If another, third module, Zvezda, is launched in July as planned, there will be at least four more space flights to the station by the end of the year - three by US shuttles and one by a Russian Soyuz spacecraft.
Work to finish the giant space laboratory will require some 40 space missions between now and 2005.
Spacewalk
The ISS will eventually house six and seven-member crews that will rotate after stays of about five months each, with the first crew, comprising one US astronaut and two Russians, to arrive late this year.
The space station, a joint venture involving the United States, Russia, Europe, Japan and Canada, is expected to cost $60bn when completed, perhaps by 2005.
It will be one of the brightest objects in the evening sky and have as much pressurised living space as a Boeing 747 aircraft.
Earlier on Monday, two Atlantis crew members completed an almost seven-hour spacewalk, repairing and modifying the outside of the ISS's two modules in six hours and 44 minutes.
Atlantis, which blasted off on Friday after several delays, is schedule to return to Earth on 29 May.