Extracellular and intracellular recordings were made from single cells in the pontine nuclei (PN) of the cat. PN cells were identified by antidromic invasion from the cerebellum by stimulating either the brachium pontis (BP) or the white matter near the cerebellar nuclei. The cerebrally-induced impulses excited PN cells postsynaptically with a monosynaptic latency. Both fast and slow conducting cortical fibres contributed to the corticopontine excitation, so that the latency varied over a wide range. Measurements of the latencies for antidromic and corticopontine excitation and of the distances between stimulated sites permitted the calcuation of conduction velocities of PN cell axons and of their cortical input fibres. PN cells with fast conducting axons received convergence from both fast and slow cortical fibres, whereas PN cells with slow axons were innervated only by slow cortical fibres. The majority of PN cells were also excited by stimulating the medullary pyramid through collaterals of the pyramidal tract. Evidence of abundant pyramidal collaterals was provided by the collision technique. The functional role of the PN is discussed in connection with the cerebro-cerebellar loop circuits.