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Notes: Summary (1) Spikes of neurons in the medial and descending vestibular nuclei were recorded extracellularly and their responses to stimulation of the interstitial nucleus of Cajal (INC) were studied in cerebellectomized cats under chloralose anesthesia. Stimuli applied in the ipsilateral INC excited 37% of neurons that did not exhibit spontaneous activity. About 84% of spontaneously discharging neurons were influenced by the INC; typical responses were excitation (35%), inhibition (22%) and excitation followed by inhibition (27%). Of the neurons that were excited, 24% fired monosynaptically. Such monosynaptic activation was evoked by stimulating the INC and midbrain medial longitudinal fasciculus (MLF), but was not evoked by stimulating the lateral midbrain reticular formation. Polysynaptic excitation or inhibition was evoked more widely, but the lowest threshold points were within the INC. Stimulation of the contralateral INC also evoked polysynaptic excitation or inhibition. However, the frequency of occurrence of the evoked responses was significantly smaller compared to the ipsilateral responses. (2) Intracellular recordings revealed that some medial and lateral vestibular neurons received monosynaptic excitatory postsynaptic potentials (EPSPs), others received polysynaptic EPSPs or inhibitory postsynaptic potentials (IPSPs) from the ipsilateral INC. The minimum latency for the IPSPs suggests that the pathway is at least disynaptic. No significant collision was observed between monosynaptic EPSPs evoked by the ipsilateral INC and contralateral vestibular nuclei. Acute lesions that damaged the pontine MLF and part of the reticular formation did not abolish monosynaptic responses of vestibular neurons by the INC. Depth threshold curves for mono- or polysynaptic responses drawn before and after the lesions were virtually similar. Antidromic thresholds of interstitio-vestibular fibers evoked from the pontine MLF showed that a great majority of these fibers run outside the MLF at the pontine level. These results control for vestibular axon reflexes, since vestibulo-interstitial fibers ascend within the MLF (cf. Gacek 1971). (3) Responses to stimulation of the INC were not different among different types of canal responding neurons; vertical and horizontal canal responding neurons received similar effects. However, canal responding neurons that received excitation from the contralateral vestibular nerve were activated more frequently by the INC than those that received inhibition (χ2 test, p〈0.01). Qualitatively similar results were obtained from vestibular neurons that had different projection sites; vestibulospinal, contralateral INC-projecting and contralateral vestibular nuclei-projecting neurons received similar effects. (4) Vestibulo-collic reflexes, studied with EMG, were modified by preceding INC stimulation. Intracellular recordings from some neck motoneurons showed that disynaptic EPSPs evoked by stimulation of the contralateral vestibular nerve were modified by preceding INC stimulation applied ipsilateral to the stimulated vestibular nerve. INC stimulation alone did not evoke any response in these motoneurons, suggesting that the interaction of the labyrinthine and interstitial effects occurred at least in part at the vestibular nuclei. (5) Some medial and descending vestibular neurons showed multiple branching, projecting to the contralateral INC, C1 or contralateral vestibular nuclei. About 34% of neurons that projected to the contralateral INC were also antidromically activated from the C1; some of them received vertical canal inputs.