The ionic mechanism of clotrimazole, an imidazole antimycotic P-450 inhibitor, was examined in rat anterior pituitary GH3 cells. In perforated-patch whole-cell recording experiments, clotrimazole reversibly caused an inhibition of the Ca2+-activated K+ current in a dose-dependent manner. The IC50 value of the clotrimazole-induced inhibition of IK(Ca) was 3 μM. In the outside-out configuration of single channel recording, application of clotrimazole (10 μM) into the bath medium did not change the single channel conductance of large conductance Ca2+-activated K+(BKCa) channels, but it suppressed the channel activity significantly. The change in the kinetic behavior of BKCa channels caused by clotrimazole in these cells is found to be due to a decrease in mean open time and an increase in mean closed time. Other structurally distinct P-450 inhibitors (e.g. ketoconazole or econazole) also effectively suppressed the amplitude of IK(Ca). Clotrimazole (10 μM) blocked both the inactivating and non-inactivating components of the voltage-dependent K+ outward current (IK(V)), but it produced a slight reduction of -type Ca2+inward current (ICa,L) without altering the current–voltage relationship of ICa,L. Clotrimazole (10 μM) also increased the firing rate of action potentials. These results provide direct evidence that clotrimazole is capable of suppressing the activity of BKCa channel in GH3 cells. Because of the non-selective inhibitory effect of clotrimazole on IK(Ca) and IK(V), this inhibition is mainly, if not entirely, due to a direct channel blockade. Thus, the present study implies that the blockade of these ionic channels by clotrimazole would affect hormonal secretion and neuronal excitability.