The magnitude of classically conditioned bradycardia was studied in 18-day-old and adult rabbits in which the cerebellar vermis had been surgically removed on either the 5th or 18th postnatal day. In the conditioning procedure, an auditory stimulus (5 s, 1000 Hz) served as conditioned stimulus (CS) and a train of electric impulses applied to the ear (100 Hz, 500 ms, 1.5 mA) was employed as the unconditioned stimulus (US). Heart rate (HR) responses developed in the operated animals during the CS-alone (orientation), and CS-US paired presentations (conditioning) were analyzed and compared with those developed in control animals. In all the experimental groups, lesions were localized to the cortex of lobules IV-VII and the underlying white matter, sparing the deep cerebellar nuclei. None of the lesioned animals showed any behavioral or somatomotor deficit. All the operated animals exhibited a normal baseline HR and a marked orienting response, both comparable with those of controls. In contrast, while the animals tested at 18 days showed a normal pattern of conditioned bradycardia, at the age of 3 months the HR conditioned response differed significantly from that observed in control rabbits: the animals that received the earliest cerebellar lesion showed a conditioned bradycardia greater than that of controls, the rabbits lesioned on the 18th postnatal day exhibited a reduced bradycardic response. These results suggest that the timing of cerebellar vermis removal, at early stages of development, represents a crucial factor in the organization of the bradycardic response in the adult.