The effects of papaverine and of the organic calcium-antagonistic agents D 600 and nifedipine on the contraction induced by noradrenaline and calcium were studied on the isolated aorta and mesenteric artery. The affinities of both agonists, given as pD2-values, were significantly higher on the aorta than on the mesenteric artery. Under our experimental conditions D 600, nifedipine and papaverine were found to act as antagonists against calcium and noradrenaline in a non-competitive fashion. In either vessel, the calcium-antagonistic activity of D 600 and nifedipine was about 1000-fold greater than that of papaverine, whereas their antagonistic activity against noradrenaline was bout 1000-times weaker, i.e. D 600 as well as nifedipine were about equipotent with papaverine. The comparison between the calcium- and the noradrenaline-antagonistic activity offers the possiblity to evaluate the specifcity of calcium antagonistic agents.