Summary: The influence of animal age on drug-induced vasodilation was investigated in the perfused rat mesentery constricted with norepinephrine. Responses to isoproterenol. a β-receptor stimulant, decreased with increasing age. Also, there was a modest decline in the relaxation produced by papaverine. In contrast. dilation of the rat mesentery by acetylcholine, his-tamine, or nitroglycerin either did not change with age or the responses became somewhat larger. Dobutamine, a myocardial β-receptor stimulant, produced a marked relaxation of the mesentery. A comparison of the action of isoproterenol and dobutamine revealed that isoproterenol stimulated vascular β-receptors, whereas dobutamine relaxed the mesentery by antagonizing the tone produced by norepinephrine. The dilation resulting from the α-receptor-blocking action of dobutamine was unrelated to animal age.