Key points Adenosine and dopamine (DA) are neuromodulators in the carotid body (CB) chemoafferent pathway, but their mechanisms of action are incompletely understood. Using functional co‐cultures of rat CB chemoreceptor (type I) cells and sensory petrosal neurons (PNs), we show that adenosine enhanced a hyperpolarization‐activated cation current Ih in chemosensory PNs via A2a receptors, whereas DA had the opposite effect via D2 receptors. Adenosine caused a depolarizing shift in the Ih activation curve and increased firing frequency, whereas DA caused a hyperpolarizing shift in the curve and decreased firing frequency. Acute hypoxia and isohydric hypercapnia depolarized type I cells concomitant with increased excitation of adjacent PNs; the A2a receptor blocker SCH58261 inhibited both type I and PN responses during hypoxia, but only the PN response during isohydric hypercapnia. We propose that adenosine and DA control firing frequency in chemosensory PNs via their opposing actions on Ih. Abstract Adenosine and dopamine (DA) act as neurotransmitters or neuromodulators at the carotid body (CB) chemosensory synapse, but their mechanisms of action are not fully understood. Using a functional co‐culture model of rat CB chemoreceptor (type I) cell clusters and juxtaposed afferent petrosal neurons (PNs), we tested the hypothesis that adenosine and DA act postsynaptically to modulate a hyperpolarization‐activated, cyclic nucleotide‐gated (HCN) cation current (Ih). In whole‐cell recordings from hypoxia‐responsive PNs, cAMP mimetics enhanced Ih whereas the HCN blocker ZD7288 (2 μm) reversibly inhibited Ih. Adenosine caused a potentiation of Ih (EC50 ∼ 35 nm) that was sensitive to the A2a blocker SCH58261 (5 nm), and an ∼16 mV depolarizing shift in V½ for voltage dependence of Ih activation. By contrast, DA (10 μm) caused an inhibition of Ih that was sensitive to the D2 blocker sulpiride (1–10 μm), and an ∼11 mV hyperpolarizing shift in V½. Sulpiride potentiated Ih in neurons adjacent to, but not distant from, type I cell clusters. DA also decreased PN action potential frequency whereas adenosine had the opposite effect. During simultaneous paired recordings, SCH58261 inhibited both the presynaptic hypoxia‐induced receptor potential in type I cells and the postsynaptic PN response. By contrast, SCH58261 inhibited only the postsynaptic PN response induced by isohydric hypercapnia. Confocal immunofluorescence confirmed the localization of HCN4 subunits in tyrosine hydroxylase‐positive chemoafferent neurons in tissue sections of rat petrosal ganglia. These data suggest that adenosine and DA, acting through A2a and D2 receptors respectively, regulate PN excitability via their opposing actions on Ih.