Proarrhythmia has been observed with the antipsychotic agent thioridazine (THIO). The mechanisms underlying these effects are unknown. The objectives of this study were 1) to characterize the effects of THIO on cardiac repolarization and 2) to determine whether lengthening of the Q-T interval could be explained by blocking major K+-repolarizing currents. Isolated, buffer-perfused guinea pig hearts (n = 32) were stimulated at various pacing cycle lengths (150-250 ms) and exposed to THIO at concentrations ranging from 300 nM to 3 microM. THIO increased monophasic action potential duration at 90% repolarization (MAPD90) in a concentration-dependent manner from 14.9 +/- 1.8 at 300 nM to 37.1 +/- 3.2 ms at 3 microM. Increase in MAPD90 was also reverse frequency-dependent; THIO (300 nM) increased MAPD90 by 14.9 +/- 1.8 ms at a pacing cycle length of 250 ms, but by only 7.7 +/- 1.2 ms at a pacing cycle length of 150 ms. Patch-clamp experiments demonstrated that THIO decreases the time-dependent outward K+ current elicited by short depolarizations (250 ms; IK250) in a concentration-dependent manner. Estimated IC50 for IK250, which mostly underlies IKr, was 1.25 microM. Time-dependent outward K+ current elicited in tsA201 cells expressing high levels of HERG protein was also decreased approximately 50% by 1.25 microM THIO. On the other hand, THIO was less potent (IC50 of 14 microM) to decrease time-dependent K+ current elicited by long pulses (5000 ms; IK5000). Under the latter conditions, IK5000 corresponds mainly to IKs. Thus, these results demonstrate block of K+ currents and lengthening of cardiac repolarization by THIO in a concentration-dependent manner. This may provide an explanation of Q-T prolongation observed in some patients treated with THIO.