Decerebrate ferrets were conditioned, using electrical stimulation of the forelimb as conditioned stimulus and periorbital stimulation as unconditioned stimulus, until they produced conditioned eyeblink responses. The latency of these was 125-250 ms. Microelectrode recordings were made from single Purkinje cells in an eyeblink controlling area in the c3 zone of the cerebellar cortex. Whereas Purkinje cells in animals, which had only received unpaired stimulus presentations, responded weakly or not at all to the conditioned stimulus, some cells in conditioned animals responded with a powerful suppression of simple spike firing. The latency of this suppression was 50-200 ms. The results support the hypothesis that classical conditioning involves plastic changes in cerebellar Purkinje cells.