1. Effects of stimulation of the cerebellar interpositus nucleus (IPN) on jaw reflexes and trigeminal motoneurons were studied along with the route responsible for the effects in ketamine-anesthetized guinea pigs. 2. Stimulation in the IPN evoked a bilateral, ipsilaterally dominant, short-latency reciprocal effect on jaw reflexes: a depression of the jaw-closing masseteric reflex and a facilitation of the jaw-opening digastric reflex. This reciprocal phase was followed by a non-reciprocal facilitatory phase. 3. Stimulation in the IPN evoked a short-latency, strychnine-sensitive inhibitory postsynaptic potential (IPSP) followed by a rebound depolarizing potential in the jaw-closing masseter motoneurons (MAMNs) and an excitatory postsynaptic potential (EPSP) in the jaw-opening anterior digastric motoneurons (ADMNs). The time course of the intracellular response of MAMNs to the IPN stimulation was similar to that of the IPN-induced effects on the jaw-closing reflex. In contrast, the duration of the IPN-induced EPSP in ADMNs was shorter than the IPN-induced facilitation of the jaw-opening reflex. 4. After the IPN neurons were lost by injection of kainic acid into the nucleus, the reciprocal effect of the IPN stimulation on the jaw reflexes could not be seen, even though the projection fibers from the trigeminal sensory nucleus to the IPN remained essentially intact. 5. Stimulation in the superior cerebellar peduncle (SP) induced the same reciprocal effect on the jaw reflexes as the IPN stimulation. A lesion of the SP virtually abolished the reciprocal effect on the jaw reflexes of stimulation in the IPN ipsilateral with reference to the SP lesion. 6. Transection of the brain stem at the level immediately caudal to the red nucleus did not affect the reciprocal effect of the IPN stimulation on the jaw reflexes. 7. We conclude that the IPN output bilaterally induces an inhibition of jaw-closing MAMNs and an excitation of jaw-opening ADMNs, oligosynaptically by way of a direct cerebelloreticular projection system via the SP.