Among the present classes of calcium channel blockers, dihydropyridine derivatives are widely used in the therapy of hypertension, angina pectoris and the other cardiovascular diseases. Since the prototype of dihydropyridine derivatives, nifedipine, does not have the optimum pharmacokinetic and pharmacodynamic characteristics, several attempts have been made to synthesize other drugs in this class with improved properties. In the present study, the effect of two new dihydropyridine derivatives, mepudipine and dibudipine, on isolated human internal mammary and rat left atrium is investigated. The two newly synthesized compounds could relax K⁺-treated internal mammary arteries. Their potencies to elicit this effect were comparable with that of nifedipine. The potency of these compounds for reducing the contraction force of rat left atrium was significantly lower than that of nifedipine. These compounds did not show any significant difference with each other in this respect. As reported in our previous study, mepudipine and dibudipine showed potent vasodilatory effects. Since they have weak action on the rat atrium, we conclude that the two new compounds are more vasoselective than nifedipine. Also, their effect on the internal mammary artery confirms their vasodilatory action in the human artery.