Harper has been recovering from left knee bursitis since late May.
On Saturday night, he faced off against Orioles left-hander Wei-Yin Chen, also on a rehab assignment.
Harper went 0-for-3 with a walk, a run and a pair of strikeouts.
Bowie won the game 6-4.
Harper struck out looking against Chen with a man on first base in the first inning. Leading off the fourth, he walked on a 3-2 count. He scored a run in the four-run fourth inning for the Senators.
In the fifth, Harper popped out to the shortstop. In the eighth inning, he struck out looking again.
Harper is expected to return to the Nationals in time for Monday’s series opener against the Brewers.