Although sigma ligand haloperidol is known to affect repolarization in heart, its effect on potassium currents in cardiomyocytes has not yet been studied. We analyzed the effect of 1 micromol/l haloperidol on transient outward K(+) current (I(to)) in enzymatically isolated rat right ventricular cardiomyocytes using the whole-cell patch-clamp technique at room temperature. Haloperidol induced a decrease of amplitude and an acceleration of apparent inactivation of I(to), both in a voltage-independent manner. The averaged inhibition of I(to), evaluated as a change of its time integral, was 23.0+/-3.2% at stimulation frequency of 0.1 Hz. As a consequence of slow recovery of I(to) from the haloperidol-induced block (time constant 1482+/-783 ms), a cumulation of the block up to about 40% appeared at 3.3 Hz. We conclude that haloperidol causes a voltage-independent block of I(to) that cumulates at higher stimulation frequencies. Based on the computer reconstruction of experimental data, a block of I(to)-channels in both open and open-inactivated states appears to be likely mechanism of haloperidol-induced inhibition of I(to).