Responses to catecholamines were studied in isolated helical muscle strips of large and small coronary vessels and in isolated perfused coronary arteries from the dog. Epinephrine and norepinephrine in concentrations well below those normally present in the blood uniformly caused relaxation of small coronary vessels. Norepinephrine was much more potent than epinephrine. The relaxation was reversibly blocked by the beta adrenergic blocker, nethalide. During this blockade, catecholamines either were inactive or produced a slight contraction. Strips contracted by angiotensin, blood or KCl, as well as those completely depolarized by K 2 SO 4 , were relaxed by catecholamines. Strips taken from large coronary vessels, unlike those from small vessels, were, in some cases, contracted by catecholamines; in others, after a transient contraction, they were relaxed. Contraction was blocked by Dibenzyline. This difference in behavior of large and small vessels may account for some of the contradictions found in the literature concerning the response of coronary vessels to catecholamines.