Lidoflazine, flunarizine, and R 51 469 antagonized contractions of isolated canine coronary arteries caused by depolarizing solution or 5-hydroxytryptamine, and the further increase in tension caused by hypoxia during such contractions. The 5-HT2-antagonist ketanserin, but not methysergide, antagonized contractions induced by 5-hydroxytryptamine, but not those by high K+; it reduced further increases in tension caused by hypoxia only during contractions caused by 5-hydroxytryptamine. Augmentation by hypoxia of coronary vasoconstriction appears to be a Ca2+-dependent phenomenon.