Mike Fossum and Ron Garan were to change out a nitrogen tank assembly, set up TV equipment and take a thermal cover off Japan's robotic arm, the National Aeronautics and Space Administration said.
The spacewalk lasted about 6½ hours, the Houston Chronicle reported.
Last week, Discovery's seven astronauts delivered a $1 billion Japanese science module and began the installation with spacewalks and robot arm operations.
The rotational device turns solar panels to track the sun as the station circles Earth.
Japan Aerospace Exploration Agency expert Akihiko Hoshide and Mission Specialist Karen Nyberg were responsible for helping Fossum and Garan with the station's mechanical arm, Canadarm2, the newspaper said.