In recent experiments Montarolo et al. (1982) have shown that reversible inferior oliver (IO) inactivation of up to 90 s induces a marked increase of the Purkinje cell (PC) simple spike (SS) frequency which lasts as long as the disappearance of the complex spikes (CS). The present work was aimed at studying how long this SS frequency increase lasts. We recorded the electrical activity of 724 PCs from rats (Nembutal anaesthesia 20-40 mg/kg) in which the IO was destroyed by means of cryocoagulation, electrocoagulation and 3-acetylpyridine injection. The SS activity was followed up to 1 month from the lesion. The average SS frequency of the first 5 days is 52.7 (+/- 22.6 S.D.). The difference from the control (22.6 +/- 13.7 S.D.) is highly significant (P less than 0.001; Mann-Whitney U-test). Later the SS discharge shows a gradual decrease, to reach a value (24.2 +/- 12.1 S.D.) similar to the control at the 4th week. Our results demonstrate that the increase of the SS activity, due to the IO lesion, undergoes an adaptation. The time course of such an adaptation is well correlated with the metabolic changes in the intracerebellar nuclei, following an IO permanent lesion, described by Bardin et al. (1983).