The peripheral control of breathing is mediated by O2-sensitive carotid body (CB) type 1 cells, which express multiple neurotransmitters including the monoamines, dopamine and serotonin (5-HT). Whereas dopamine has been extensively studied, 5-HT has received little attention. Here, to elucidate the role of 5-HT in CB chemotransmission, we used perforated-patch recording from rat type 1 cell clusters and co-cultured petrosal (afferent) neurones. 5-HT induced action potentials and/or membrane depolarization associated with a conductance decrease in approximately 40% of recordings from type 1 cells (n = 78/192). These responses were markedly inhibited by the 5-HT2 receptor antagonist ketanserin (10-50 microM) and by the protein kinase C (PKC) inhibitor chelerythrine (50 microM). The PKC activator 1-oleoyl-2-acetylglycerol (OAG; 50 microM) mimicked the 5-HT-induced depolarization, and the combined effects of 5-HT and OAG were non-additive. The 5-HT-induced responses reversed near the potassium (K+) equilibrium potential (at approximately -82 mV; EK = -83 mV), suggesting inhibition of a resting K+ conductance. In type 1 cells (n = 7), voltage-activated outward K+ current was also inhibited by 1-50 microM 5-HT, an effect that was prevented by PKC inhibitors (chelerythrine and NPC 15437) and mimicked by OAG; the outward K+ current inhibited by 5-HT appeared to be predominantly a Ca(2+)-dependent K+ current. The 5-HT2 receptor blockers ketanserin and ritanserin reversibly inhibited spontaneous action potentials and the hypoxia-induced receptor potential recorded from clustered type 1 cells. Moreover, these blockers reversibly inhibited the hypoxic chemosensory response recorded postsynaptically in petrosal neurones that functionally innervated type 1 clusters in co-culture. RT-PCR and confocal immunofluorescence techniques revealed 5-HT2a receptor expression in rat CB type 1 cells. These results suggest that release of endogenous 5-HT regulates CB chemoreceptor function presynaptically, by a positive feedback mechanism involving autocrine-paracrine stimulation of 5-HT2a receptors and PKC modulation of resting and Ca(2+)-dependent K+ conductances.