In contrast to nifedipine-like calcium antagonists, calcium agonistic 1,4-dihydropyridines have vasoconstricting and positive inotropic properties. BAY K 8644 has become the prototype of this new class. Its enantiomers show opposite effects on the calcium channel: one acts as a calcium agonist with the pharmacological profile of the racemic compound, its antipode has calcium antagonistic effects at 10 times higher concentrations. Voltage clamp studies reveal calcium current increasing effects at 3 X 10(-8) mol/l BAY K 8644, while the current is reduced at 3 X 10(-6) mol/l. Analysis of the calcium current activation and deactivation kinetics shows that BAY K 8644 leaves the mean closed times of the calcium channel unchanged while it increases the mean open times. From these data a reaction model of drug action is derived, suggesting that BAY K 8644 binds only to the open state of the Ca-channel.