Recently, the novel dihydropyridine derivates YC-170, CGP 28392, and BAY K 8644 have been reported to act in the opposite way to Ca2+ -entry blockers. We have found that these compounds inhibit the binding of [3H]nitrendipine on guinea pig heart membranes (Ki: 6 nM BAY K 8644, 115 nMCGP 28392 and 690 nM YC-170). Like those of nifedipine (Ki 1 nM), the curves had slopes close to unity, and. unlike those of some nondihydropryridine Ca2+ antagonists, were not altered in the presence of diltiazem, indicating a competitive interaction at dihydropyridine-sensitive sites. In isolated guinea pig atria, these agents exerted positively inotropic effects similar in their potency ratio to those observed in the binding experiments (BAY K 8644 1, CGP 28392 1:17, YC-170 1:600). The maximum inotropic effects of BAY K 8644 and CGP 28392, and of YC-170 corresponded respectively to two-thirds and one-third of those induced by isoprenaline or extracellular Ca2+ In the isolated rat mesenteric artery, perfused with a depolarizing solution, vasoconstrictor Ca2+ dose-response curves are shifted to the right by nifedipine. By contrast, BAY K 8644 and CGP 28392 caused a distinct leftward shift of the Ca2+ dose-response curves, at concentrations of 3–300 nM and 30–300 nM, respectively, and YC-170 a marginal shift at concentrations of 200–2000 nM, i.e., similar ranges to their inhibitory effects on [3H]nitrendipine binding. At higher concentrations, all three compounds produced Ca2+ -antagonistic effects. These results indicate that the compounds act at dihydropyidine-sensitive sites and exert partial agonistic activities in vascular and myocardial tissue.