The participation of basal ganglia in motor initiation was studied in six cats operantly trained to perform a ballistic flexion movement, triggered after a brief sound in a simple reaction time condition or delayed after the same sound in the presence of a tone cue. The activity of 356 neurons was recorded in the putamen and in the pallidum (globus pallidus and entopeduncular nucleus). A total of 19.4% of the neurons were not related to the conditioned flexion movement: they were either unrelated to the task (10.1%) or related to other periods of the motor performance such as trial beginning or reward delivery (9.3%). About 60% of the remaining neurons--defined as task-related--exhibited changes in firing rate that occurred, in the reaction time condition, less than 100 ms after the go signal and therefore began prior to movement onset. For most neurons, in the delayed condition, these early changes were absent, suggesting that their occurrence in the reaction time condition was not a sensory response but rather was related to the movement initiation. In addition, for many neurons these changes shifted in time, remaining time-locked to the movement. Correlations between these early changes in activity and motor parameters were demonstrated, suggesting that these changes were movement-related. For most neurons the firing levels observed during intertrial intervals and during foreperiod were similar. The mean discharge rate during the foreperiod was 19.2 impulses/s. Three patterns of activity were observed before movement: increases in discharge rate (61% of task-related neurons), transient decreases followed by increases (11%), or prolonged decreases (28%). Only minor differences were found between the characteristics of the populations of neurons recorded in the three sites under study: on average the neurons recorded in the globus pallidus were more active than the neurons recorded in the putamen or in the entopeduncular nucleus. The fact that, for certain neurons, the changes of activity prior to movement were different in reaction time condition and in delayed condition showed that the pattern of activity preceding movement might depend on the temporal requirements for motor initiation. Taken together with the motor effects obtained in the same task following GABA-receptor activation with muscimol microinjections in these structures, the present results suggest that putamen and pallidal neurons participate in the initiation of the conditioned movement under study.