The effect of iontophoretic application of serotonin (5-HT) was studied in neurons of the cochlear nucleus in the rat. 5-HT inhibited the spontaneous activity in 71%, and the tone-evoked activity in 32% of the neurons. We also observed an excitatory effect, with a longer latency than that of the inhibition, in 40% of the neurons. In some neurons 5-HT had both inhibitory and excitatory effects. Neurons with different response types seem to have different sensitivities to 5-HT. As the effects of 5-HT were generally weaker than those of other putative neurotransmitters, it probably has only a small modulatory influence on auditory processing.