BQ-123 is extensively used as an antagonist at endothelin (ET) receptors, having selectivity at the ET A receptor subtype. In this study, the effects of BQ-123 per se on action potentials, l-type calcium currents, and potassium currents, were examined in ventricular cardiomyocytes isolated from adult, male, New Zealand White rabbits, using the patch-clamp technique. BQ-123 (1μM) increased (P < 0.02) the duration of the action potential to 267 ± 36 ms from a control duration of 228 ± 30 ms. BQ-123 did not have any effect on the inward rectifier or transient outward potassium currents, but increased (P < 0.02) the l-type Ca 2 + current to -2.76 ± 0.3 nA from a control value of -2.45 ± 0.28 nA. The increases in both duration of the action potential and l-type Ca 2 + current were reversed upon washout (233 ± 28 ms and -2.32 ± 0.31 nA, respectively) and were not different from the control values in the absence of BQ-123. In contrast, the endothelin receptor antagonists, BQ-788, PD155080 and PD145065 (1-10 μM) did not affect the l-type Ca 2 + current. These results indicate that, unlike PD155080, BQ-788 and PD145065, the conventional ET A receptor-selective antagonist, BQ-123, exerts a unique positive effect on the l-type Ca 2 + current in ventricular cardiomyocytes isolated from rabbit myocardium. The mechanism of action of BQ-123, therefore, is not confined to ET receptor antagonism.