Stimulation with alternating current at the round window causes an accelerated uptake of extracellular HRP by endocytotic vesicles in inner hair cells at the base of the stimulated cochlea, near the site of the electrode, but does not noticeably affect uptake into outer hair cells. After unilateral electrical stimulation, efferent endings on outer hair cells at the cochlear base show increased vesicular and endosomal labelling in both the ipsilateral and contralateral ears. It is concluded that round-window electrical stimulation increases afferent synaptic activity in inner hair cells near the electrode site, and also increases synaptic activity in OHC efferent endings, not only of the ipsilateral cochlea but also of the corresponding region of the contralateral cochlea. Rapid diffuse cytoplasmic labelling of inner and outer hair cells also occurs sporadically in both stimulated and unstimulated cochleae. However, in efferent terminals, diffuse labelling is restricted to stimulated animals, and occurs bilaterally, in corresponding basal regions of the cochlea.