Intracellular microelectrode recordings were made from single neurones of the ventral nucleus of trapezoid body and rostral periolivary regions in the rat auditory brainstem, using in vitro slice techniques. Bath application was used to examine the effects of putative neurotransmitters and neuromodulators on cell responses to constant depolarizing current pulsse. Noradrenaline exerted excitatory effects (increased firing rate) that were probably mediated by cr-receptors, whereas inhibitory effects (decreased firing rate) were probably mediated by beta-receptors. Serotonin also produced either excitatory or inhibitory effects in different cells, Of the neuroactive peptides, substance P and enkephalin were especially potent. Substance P was found to be exclusively excitatory and enkephalin was exclusively inhibitory. Choleycystokinin exerted either inhibitory or excitatory effects in a small percentage of cells, Somatostatin had only very weak or non-existent effects. These effects were able to be elicited under conditions of synaptic blockade, indicating they they were mediated by direct action on the cells in question. Most effects on firing rate were accompanied by either depolarization or hyperpolarization of the resting membrane potential although in many cases this change in membrane potential was small. Changes in cell access resistance were also relatively difficult to detect, but in the case of both noradrenaline and substance P, clear increases in cell access resistance were recorded in a number of cells. These could be obtained in the presence of tetrodotoxin, again indicating a direct action of these substances rather than an indirect action mediated via synaptic connections. Although the exact mechanisms of action remain to be investigated in each case, it is clear that neurones in this region of the auditory brainstem are potentially subject to a wide variety of modulatory influences that could be important in auditory processing.