1. The contractile and electrical responses to acetylcholine (ACh) in isolated segments of guinea-pig and rabbit coronary arteries were compared to those of the putative adenosine 5'-triphosphate (ATP)-dependent K+ channel opener, BRL 38227. 2. Both ACh and BRL 38227 produced concentration-dependent relaxation of vessel segments contracted with the H1-receptor agonist, 2-(2-aminoethyl)pyridine. 3. An IC90 of either vasodilator also produced 17-20 mV of hyperpolarization of the guinea-pig coronary artery. 4. Glibenclamide (1-35 microM) depolarized the guinea-pig coronary artery by 8-12 mV and antagonized BRL 38227- but not ACh-induced relaxation and hyperpolarization. 5. In the guinea-pig coronary artery, the K+ channel blockers phencyclidine (PCP, 100 microM), tetraethylammonium (TEA, 10 mM) and scorpion venom (8.7 micrograms ml-1) all significantly reduced ACh-induced relaxation and hyperpolarization whereas only PCP was an effective antagonist of both relaxation and hyperpolarization with BRL 38227. 6. Similar effects of glibenclamide and scorpion venom on ACh- and BRL 38227-induced relaxation were observed in the rabbit coronary artery. 7. Apamin (3.5 microM) was without effect on either the ACh- or BRL 38227-induced relaxation in the guinea-pig coronary artery. 8. In conclusion, the actions of BRL 38227 in coronary artery are compatible with its proposed effects on ATP-dependent K+ channels. In contrast, the results with ACh suggest that some step between the initial binding of ACh to endothelial muscarinic receptors and the final relaxation of the smooth muscle depends upon the opening of Ca(2+)-activated K+ channels.