A major function of the dopamine in the striatum is to control the activity of its efferent systems which contain primarily GABAergic neurons. Direct intracerebral injections of haloperidol into the corpus striatum impaired the performance of rats trained to depress a lever in a sensitive reaction time task. Rats were trained to depress a lever until the presentation of a visual conditioned stimulus and then to release the lever within a time limit of 500 ms to obtain a food reward. The increase in reaction time produced by dopamine blockade of the corpus striatum was mimicked by injection of a GABA agonist muscimol in nanogram quantities into the region of the dorsal pallidum, but not into the substantia nigra reticulata. Injections of a GABA agonist into the substantia nigra impaired performance by increasing the number of anticipatory responses (increased number of lever releases before the conditioned stimulus). These results suggest that the dorsal pallidum may play an important role in the response initiation associated with activation of the corpus striatum, and that the dorsal pallidum may form a significant part of parallel striatal outputs that have different functional significance.