Intracellular recordings were made from guinea-pig prepositus hypoglossi (PH) neurons in vitro. Neurons within this nucleus are innervated by terminals that release 5-HT (serotonin) to mediate an IPSP. Cocaine caused a concentration-dependent prolongation of that IPSP, while having no effect on either membrane potential or firing rate. Cocaine also caused an increase in the IPSP amplitude at lower concentrations (less than or equal to 1 microM) and a decrease at higher concentrations. The selective 5-HT uptake inhibitor fluoxetine also prolonged the IPSP duration but depressed the amplitude at all concentrations tested. The effects of cocaine on IPSP duration can be completely accounted for by inhibition of 5-HT uptake. The depression of the IPSP is most likely due to a presynaptic effect, because cocaine augmented the response to applied 5-HT. Amphetamine (3–300 microM), unlike cocaine, changed the membrane potential. At lower concentrations, it caused a ketanserin-sensitive depolarization, while higher concentrations resulted in a spiperone-sensitive hyperpolarization. The hyperpolarization was most likely caused by the evoked release of 5-HT, while the depolarization may have been due to a direct effect of amphetamine on 5-HT2 receptors. Amphetamine also acted as a weak uptake inhibitor. The effects of cocaine, but not those of amphetamine, were observed at concentrations that are attained during self-administration.