The long term effects of the inferior olive degeneration on red nucleus activity were studied in the rat. The animals were injected with 3-acetylpyridine to produce a pharmacological destruction of the inferior olive and were then used for acute experiments at 1-2, 5-7, 14-18, 29-37, 81-110 and 236-255 days later. After degeneration of the inferior olive, there was an 'initial period' lasting for a few days, characterized by a low discharge frequency of the red nucleus neurones. A 'period of adaptation' followed during the first month, characterized by a slow recovery towards the control firing rates of the rubral units. Nevertheless, the temporal distribution of the discharges was not recovered since the firing became organized in a bursting activity. From 1 up to 8 months, the normal unit activity was not restored. The hypothesis is advanced that the suppression of the inferior olive which increases the cerebellar inhibition, produces a consequent disfacilitation of red nucleus activity which persists for a few days. Then at increasing survival times, a progressive compensation takes place without a real restoration of the initial rubral activity.