The involvement of a guanine-nucleotide-binding regulatory protein (G protein) in the relaxing responses to adenosine receptor agonists was investigated in bovine coronary vessels. Ring segments of left anterior descending artery branches were suspended in organ baths for measurement of isometric tension. The adenosine analogs, 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CAD) caused concentration-dependent relaxations of coronary rings contracted with KCl. The relaxing effects of NECA and CAD were antagonized by the adenosine receptor antagonist 8-phenyltheophylline indicating the involvement of an adenosine receptor. In a separate series of experiments, incubation with cholera toxin inhibited the relaxing responses to NECA, CAD and isoproterenol but not those produced by sodium nitroprusside. Treatment with forskolin did not reduce the relaxing responses to NECA or CAD. N-ethylmaleimide and NaF/AlCl3 caused significant inhibition of the relaxations produced by both NECA and CAD. Incubation with pertussis toxin was without effect on relaxations induced by NECA and CAD. These results provide evidence for the involvement of G protein (possibly stimulatory G proteins) in the relaxing effects mediated by the bovine coronary artery adenosine receptor.