Unrestrained cats performed ballistic forelimb flexion movements triggered by an auditory stimulus (CS) on a simple reaction time (RT) paradigm. During the variable foreperiod the subject was required to hold down a lever and to release it on presentation of the CS. The RTs ranged from 200 to 300 ms. The activity of single neurons of the ventrolateral nucleus of the thalamus (VL) was recorded bilaterally. More than 40% of the 166 units recorded in the VL contralateral to the performing limb presented, after the CS, changes of activity with a latency less than 100 ms and were classified into three types: (1) Twenty-five units had a short latency transient increase of activity 10 to 30 ms after the CS, followed by a longer increase or decrease in activity. Short latency increase as well as subsequent increase of the firing rate were not correlated to the RTs. (2) Twenty-nine units showed a 40-60 ms latency increase of activity which lasted long enough to continue during the forelimb movement. These units displayed a correlation between the RTs and the mean firing rate measured in the 40-100 ms period after the CS. The more the cells were activated, the shorter the RTs. (3) Fifteen units presented a reciprocal pattern of discharge with respect to the type (2) units. The firing rate decreased with latencies ranging from 20 to 90 ms after the CS. Only 14,5% of the 96 units recorded in the VL ipsilateral to the performing limb presented changes of activity starting in the 100 ms period following the CS. Background firing levels as well as phasic activity were rather low compared to those observed contralaterally. Sixteen units showed burst activity while the cat was performing but burst pattern was not time-related to the task. In an unconditioned animal, a very low level of activity and an absence of modulations were observed in both VLs.