In an in vitro preparation of the intact carotid body (CB) of the rabbit, adenosine (100 μm) inhibited hypoxia-induced catecholamine release by 25%. The specific A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1 μm) prevented the inhibition and increased the response to hypoxia further. In isolated chemoreceptor cells from the same species, adenosine inhibited voltage-dependent Ca2+ currents by 29% at 1 μm (concentration producing half-maximal inhibition, IC50 = 50 nM). This inhibition was mimicked by R(–)N6-(2-phenylisopropyl)-adenosine and 2-chloroadenosine (1 μm), two purinergic agonists poorly active at the intracellular (‘P′) site, and persisted in the presence of dipyridamole (a blocker of adenosine uptake; 1 μm) and was fully inhibited by 8-phenyltheophylline (10 μm). The A1 antagonists DPCPX (10 μm) and 8-cyclopentyl-1,3-dimethylxantine (0.1 μm) inhibited the effect of adenosine by 93% (IC50 = 0.14 μm) and 59%, respectively. The inhibition of the Ca2+ current (ICa) was reduced by nisoldipine (an L-type Ca2+ channel antagonist) by nearly 50%, and was unaltered by omega-conotoxin GVIA, a blocker of N-type Ca2+ channels. Adenosine did not affect the voltage-dependent Na+ current (INa) or K+ current (IK.). We conclude that adenosine A1 receptors are located in chemoreceptor cells and mediate the inhibition of L-type Ca2+ channels and thereby the release of catecholamines produced by hypoxia. The data also indicate that endogenous adenosine acts as a physiological negative modulator of the chemoreceptor cell function. The previously reported excitatory action of adenosine on the activity of the sensory nerve of the CB is discussed in terms of a balance between the inhibition mediated by A1 receptors and the excitation mediated by A2 receptors.