Postnatal development of mossy fiber afferents from the vestibular and the visual system to the vestibulocerebellum was studied electrophysiologically and morphologically. In kittens anesthetized with pentobarbital sodium and N2O plus halothane, extracellular simple and complex spikes of Purkinje cells were recorded in the flocculus, nodulus and uvula. In the flocculus, stimulation of the VIIIth, but not the optic nerve, evoked simple spike responses with a latency of 16 ms at the day of birth which decreased to 5 ms by day 15 (short latency group). On the other hand, another group of simple spike responses with much longer latencies (50-80 ms) began to be elicited on day 7 via both the optic and VIIIth nerves. The latency decreased to 24 ms by day 15 and 10 ms on day 30. These latencies further shortened with development to the adult latency value (3-5 ms). Simple spike responses of the short latency group were also evoked in the nodulus and uvula from the VIIIth nerve with a slightly longer latency than that in the flocculus (23 ms on day 3 and 12 ms on day 17). Because of the immaturity of granule cells in early postnatal days, short latency simple spike responses from the VIIIth nerve suggested the direct synaptic connection of vestibular mossy fibers with Purkinje cells. Horseradish peroxidase was injected into the white matter of the flocculus, nodulus and uvula in slice preparations. Mossy fibers labeled with horseradish peroxidase showed fine branches extending to reach Purkinje cell somata from mossy swellings in the internal granular layer during days 2-20. Electron microscopy showed that the labeled mossy fibers made intimate contacts with Purkinje cell somata and the terminals contained many round synaptic vesicles. Pre and postsynaptic densities were occasionally found. After day 20, direct mossy fiber connections with Purkinje cells could not be observed. During days 7-20, these direct connections, as well as mossy fiber-granule cell connections could be observed. It was demonstrated that during early postnatal development, vestibular mossy fibers temporarily make direct contact with Purkinje cells, through which impulses could be transmitted to elicit simple spikes in Purkinje cells.