The effects of reversible pharmacological manipulation of the neuronal activity in the pedunculopontine tegmental nucleus (PPTg) on the performance of a conditioned movement was studied in two freely moving cats. The microinjections were given in regions where, in the same subjects, we had previously identified neurons with context-dependent early activity after a trigger stimulus and with reinforcement-related activity. The subjects were conditioned to perform a forelimb-flexion movement controlled by a simple reaction-time task. In addition, one subject was trained to execute the same flexion movement, but delayed after the trigger stimulus. Food pellets were used as the reinforcer. Lidocaine injections (1 microl of 2% solution, injected over a 6-min period) induced a transient arrest of performance within minutes. The cessation of performance could be preceded by behavioral signs such as meowing, attempt to escape from the experimental booth, licking, or stereotyped posture. No rotational behavior could be observed. The effects of lidocaine could be mimicked in one subject by an extinction procedure. Muscimol injections (two injections of 0.2 microg in 1 microl, tested in one subject) also induced arrest of performance, but the return to pre-injection level of performance could not be obtained within the time of the test session. The quantitative analysis of reaction times and of inter-trial intervals showed that altering PPTg activity affected inter-trial intervals, but only slightly affected the reaction times. It is speculated that the PPTg is involved in the reinforcement process related to selecting the appropriate motor program.