OBJECTIVE To explore the effect and underlying mechanism of promethazine(PMZ) on proarrhythmia in guinea pigs. METHODS ① InvivoECG recordings were made to analyze effects of jugular intravenous(iv)injection of PMZ on ECG in guinea pigs. PMZ was injected in this order:3.83→7.67→15.33→38.33 mg·kg-1 cumulatively. ② In vitroECG recordings were made to analyze effects of PMZ on ECG in isolated hearts of guinea pigs. PMZ was perfused in such order:0. 1 → 1 → 10 →50 μmol·L-1 . ③ L-type Ca2+ currents from ventricular myocytes in guinea pigs were recorded to investi-gate the PMZ's blocking effect. PMZ was perfused in such order:0.1→1→10→50 μmol·L-1→washout.④ hNav1.5 and hERG currents were recorded to investigate the PMZ's blocking effects. PMZ-perfused in such order:1→3→10→30 μmol·L-1 for hNav1.5 current analysis,and 0.3→1→3→10 μmol·L-1 for hERG current analysis. RESULTS ① PMZ(15.33 mg·kg-1 )significantly prolonged QRS intervals in guinea pigs invivoECG(P﹤0.05). PMZ(38.33 mg·kg-1 )prolonged QRS,QTc,and P-R intervals but reduced the heart rate( P﹤0.05). PMZ(10 μmol·L-1 )reduced the heart rate of isolated guinea pig hearts. PMZ 50 μmol·L-1 prolonged QRS and QTc intervals and further reduced the heart rate(P﹤0.05).③ PMZ inhibited the L-type Ca2+ current from ventricular myocytes in guinea pigs in a concentration-dependent manner with the lC50 of(8.9±1.0)μmol·L-1 . ④ PMZ inhibited the hNav1.5 and hERG currents in a concentration-dependent manner with the lC50 of 6.1±1.5 and(1.6±0.2)μmol·L-1 ,respectively. CONCLUSION PMZ might cause arrhythmia at overdoses and incombination with other drugs which have potential blocking effect on /Na ,Ca2+ and /kr currents. The proarrhythmic effect of PMZ might be mediated by the blocking effect on /Na ,Ca2+ and /kr currents.