The action of the blood clotting enzyme thrombin on single channel and whole cell Ca(2+)-currents was studied in isolated mammalian cardiac myocytes. Thrombin, at a concentration of 10(-8) mol/l, increased the Ca(2+)-channel activity in cell-attached patches. The mean open probability of the channel was enhanced, while the number of sweeps without openings, which reflects the availability of the channel, was significantly reduced. Neither the single channel conductance nor the activation curve were affected by thrombin. Thrombin was added to the bath solution, and its effect is therefore indirect and probably mediated via a second messenger. However, thrombin did not affect whole-cell Ca(2+)-currents, whereas a beta-adrenergic stimulation in the same cell increased the Ca(2+)-current. It is concluded that thrombin affects an intracellular mechanism for Ca2+ channel current regulation, which is still unknown and which is rapidly lost during conventional whole-cell Ca2+ current measurements.