Serotonin (5-HT) neurons in the dorsal (DRN) and median (MRN) raphe nuclei, and dopamine (DA) neurons in the substantia nigra (SN) were recorded extracellularly in the anesthetized rat. Compounds which have a relatively high affinity for the 5-HT1A or 5-HT1B subtypes of the 5-HT1 receptor were administered and their effect on the firing rate of the monoamine cells was determined. 5-HT1A ligands were more potent in inhibiting impulse activity in the DRN than in the MRN, but had little effect in the SN. In contrast, 5-HT1B ligands increased the firing rate of MRN 5-HT units at low doses, and were also effective inhibitors of DA cell firing in the SN. These results could be correlated with recently described differences in the distribution of the 5-HT1A and 5-HT1B receptor subtypes, and were interpreted as indicating possible functional differentiation between these subtypes. In particular, agonist activity at the 5-HT1B autoreceptor site may decrease 5-HT release, suggesting a presynaptic locus for this receptor in the somatodendritic region. The site also appears to be implicated in 5-HT modulation of nigral DA impulse flow.