The relaxing effect of dopamine on isolated coronary vessels and the activating effect of dopamine on large-conductance, calcium-activated potassium channels (BKCa) in the membrane of coronary myocytes were investigated with isometric tension recording method and patch-clamp technique. Tension studies demonstrated that dopamine relaxed prostaglandin F2α-induced contraction of porcine coronary arteries in a concentration-dependent manner, but it failed to relax high [K+] precontracted arteries. In cell attached patch experiments, dopamine caused a significant increase in the mean opening probability of the BKCa channels. The effect of dopamine was not blocked by propranolol but was completely prevented by SCH23390, a selective DA1 antagonist. These results demonstrate that dopamine relaxes prostaglandin F2α-induced contraction of porcine coronary arteries via activation of DA1 receptors which causes stimulation of BKCa channel activity.