The right-hander threw 25 pitches -- fastballs and breaking pitches -- without any problems.
The Nationals are hoping that Mattheus will be ready to pitch in a big league game by Sept. 20. In the meantime, Mattheus will play catch again on Monday in New York before throwing another bullpen on Tuesday.
"There is not going to be anything holding me back from getting back out there," Mattheus said. "I don't know how many innings it will be, but I think I'll be out there pretty soon."
Before getting injured, Mattheus was one of manager Davey Johnson's reliable relievers, going 2-2 with a 2.40 ERA in 32 games.