The vasomotor responses in vitro to magnesium, nitroglycerin and verapamil were investigated in human coronary arteries. In order to examine possible species differences in reactivity to these agents, experiments were performed also on cat and rat coronary arteries. Potassium (124 mM) regularly produced stable contractions suitable for experiments with dilator agents. The order of potency for eliciting relaxation was the same in all three species; verapamil greater than nitroglycerin greater than magnesium. Maximum relaxation induced by nitroglycerin or magnesium was significantly lower in arteries from cat as compared to that obtained in coronary artery segments from man and rat. Spontaneous rhytmic activity was often present in human coronary arteries but never in arterial segments from cat or rat. The rhytmic activity was frequently enhanced by the addition of prostaglandin F2 alpha (3 microM) to the tissue bath, on the other hand the rhythmic activity was depressed, or even abolished, by potassium (124 mM), magnesium (1.2-13.2 mM), nitroglycerin (2.2 X 10(-5) M) or verapamil (10(-8)-10(-7) M). The magnitude of the vasomotor response of feline coronary arteries to nitroglycerin or verapamil was dependent on the extracellular concentration of magnesium; in the presence of a high concentration of magnesium (4.4 mM) the dilator effect of nitroglycerin was enhanced while that of verapamil was slightly depressed. The dilator activity of the two agents was not changed by incubation of the vessel segments in a magnesium-free medium as compared to that obtained in the standard (1.2 mM Mg) buffer solution.