Responses evoked by electrical stimulation (auditory division of the VIIIth nerve sectioned chronically) and natural stimulation of the vestibular apparatus were recorded in the anterior and posterior cerebellar vermis of cats anesthetized with Ketamine or Nembutal. Under Ketamine the functional state of the cerebellar cortex was similar to that of the decerebrate or encéphale isolé preparation. 2. Vestibular-evoked responses were found bilaterally throughout the vermis (lob. I-X) and parts of pars intermedia and were, for the most part, mediated via the mossy fiber-granule cell pathway although natural stimulation occasionally evoked climbing fiber responses in Purkinje cells. 3. Lesion and stimulation experiments suggested that the polysynaptic potentials recorded in the dorsal folia of the anterior and parts of posterior vermis were mediated, at least in part, by the lateral reticular nucleus. Potentials recorded in the deeper folia often had shorter latencies and were probably mediated by primary and/or secondary vestibular fibers. Studies with horseradish peroxidase (injections in lob. V and VI) supported these notions. 4. An analysis of Purkinje cell responses to sinusoidal rotation and steps of angular acceleration or velocity indicated that P-cells in these regions signalled angular head velocity in the mid-frequency range. Single canal responses as well as multi-canal convergent P-cell responses were found. Purkinje cells also responded to static head displacement.
