Motor activation time (MAT), considered in the present work as the act
ual intracerebral processing delay during a reaction time (RT) task, w
as assessed in 17 patients with Parkinson's disease (PD) and in 7 age-
matched healthy volunteers. MAT was calculated by subtracting from the
premotor RT the afferent and efferent times obtained by sensory and m
otor evoked potentials. When compared with healthy volunteers, patient
s exhibited significantly (p < 0.02) prolonged MATs. In a smaller grou
p of 9 PD patients MAT and the auditory P3 event related potential wer
e assessed while on antiparkinsonian treatment and after a 12-hour wit
hdrawal period. During the off medication condition patients showed a
significant slowing (p < 0.01) of MAT values without any remarkable ch
ange in P3 latency or amplitude. These results suggest that MAT slowin
g indicates an abnormal function of the dopaminergic mechanism involve
d in the initiation of movement which is not related to changes in the
arousal or cognitive state.