In Wistar rats we have studied the effect of inferior olive lesion or activation on the threshold of a flexor reflex elicited by a nociceptive stimulus applied to the hindpaw. When the inferior olive is lesioned by means of 3-acetylpyridine, the threshold value is significantly decreased. A recovery occurs in 3-4 weeks. When the inferior olive is activated by means of harmaline, the threshold value is significantly increased. These experiments suggest the inferior olive activity exerts an inhibitory effect on flexor reflex activity. The recovery of the threshold value depends, probably, on the plastic reorganization of the cerebellar circuits, which occurs after inferior olive lesion.