Axons of olivocochlear neurones in the superior olivary complex terminate on hair cells of the cochlea, reducing the sensitivity to sound. These axons also have collateral branches to neurones in the cochlear nucleus, the first processing centre in the brainstem. Anatomical data show that these collaterals terminate mainly in the granule cell area but their precise neuronal targets and the effects they might have are unknown. We have studied the effects of these collaterals in guinea pigs, by electrically stimulating the olivocochlear axons at the floor of the IVth ventricle while recording single neurone responses in the cochlear nucleus. We eliminated the peripheral effects of olivocochlear stimulation either by destruction of the target receptor cells using chronic administration of kanamycin, or by acute perfusion of the cochlea with strychnine, a specific blocker of the postsynaptic receptors. Electrical stimulation of the olivocochlear axons in normal animals caused a variety of effects on cochlear nucleus neurones. In some neurones, there was suppression of spontaneous firing and a reduction in sensitivity to sound, while in others there was an excitatory effect of olivocochlear axon stimulation. When the peripheral olivocochlear action was eliminated, we still found both inhibition and excitation in the cochlear nucleus. These results show that the effects of olivocochlear stimulation on cochlear nucleus responses are not a simple passive reflection of peripheral changes but are a result of complex interactions between peripheral suppression of afferent input and collateral-mediated excitation and possibly also inhibition.