Several lines of evidence have implicated the subthalamic nucleus (STN) in behaviors that require precise temporal control. In order to investigate this possibility further, we trained rats to perform a peak-interval (PI) timing task [S. Roberts, Isolation of an internal clock, J. Exp. Psychol. Anim. B 7 (1981) 242-268] with two durations (10 s and 40 s), and then lesioned the STN by microinjection of ibotenic acid. Lesioned animals were able to maintain precise temporal control, yet were unable to inhibit operant responses late in the trial, at times that were unlikely to yield reinforcement. These results indicate the involvement of the STN in impulsive or perseverative response inhibition, but not in temporal processing.