Nationals third baseman Ryan Zimmerman was activated from the disabled list and is in Tuesday's lineup against the Cardinals.
To make room on the 25-man roster, right-hander Craig Stammen was optioned to Triple-A Syracuse.
Zimmerman, who hasn't played in a big league game since April 9, missed more than two months of action because of a torn abdominal muscle after undergoing surgery on May 3.
Zimmerman Activated, In Nationals' Lineup