The calcium-complexing abilities of acetate, bicarbonate, lactate, succinate, and citrate were quantified, and were found to differ substantially. Acetate was a more potent vasodilator than lactate, and the vasorelaxant potencies of acetate and lactate failed to correlate with their calcium-complexing ability. The vasodilator action of acetate was not abolished after correction for complexing of ionized calcium. Strips constricted with vasopressin were especially sensitive to the vasodilator effects of acetate.