It is known that hypoxia (PO2∼66–18 mm Hg), acting via unknown receptors, increases carotid body cAMP levels in Ca2+-free solutions, indicating that low PO2 activates adenylate cyclases independently of the action of the released neurotransmitters. The aim of the present work was to investigate the involvement of G proteins in the genesis of the basal level of cAMP and on the increase in cAMP induced by low P02. In carotid body homogenates, cholera toxin- and pertussis toxin-induced [32P] ADP-ribosylation of two protein bands of =42 and 45 kDa, and =39 and 40 kDa respectively; in both cases, prior incubation of the carotid bodies with the toxins reduced [32P] ADP-ribosylation by >90%. In intact carotid bodies, cholera toxin treatment increased cAMP levels more in normoxic than in hypoxic organs, indicating that hypoxia releases neurotransmitters acting on receptors negatively coupled to adenylate cyclases. Cholera toxin-treated carotid bodies incubated in Ca2+-free solution had identical cAMP levels in normoxia and in hypoxia. In pertussis toxin-treated normoxic carotid bodies the cAMP level was close to control, but in pertussis toxin-treated hypoxic carotid bodies cAMP rose to a level similar to those seen in normoxic cholera toxin-treated organs, indicating that low PO2 releases neurotransmitters acting on receptors positively coupled to adenylate cyclases. Pertussis toxin-treated carotid bodies incubated in Ca2+-free solution lost their capacity to increase cAMP in response to hypoxia, indicating that a G protein sensitive to pertussis toxin is needed for this response. This implies that the carotid bodies express a pertussis toxin-sensitive G protein positively coupled to adenylate cyclases, or that a Gs protein requiring the cooperative action of Go/Gi donated by subunits mediates the increase in cAMP level produced by hypoxia.