The antipsychotic and anxiolytic phenothiazine, cyamemazine, was investigated for its effects on the hERG (human ether-à-go-go related gene) channel expressed in HEK 293 cells and on native INa, ICa, Ito, Isus, or IK1 of human atrial myocytes. Moreover, cyamemazine and terfenadine were compared for their effects on the QT interval in anesthetized guinea pigs. Cyamemazine reduced hERG current amplitude with an IC50 value of 470 nM. Cyamemazine 1 microM failed to significantly affect INa, Ito, Isus, or IK1 amplitudes and slightly decreased ICa (18%). For comparison, haloperidol (30 nM) and olanzapine (300 nM) reduced hERG current amplitude by 44.2+/-3.9% and 49.7+/-4.2%, respectively. The cardiac safety ratio of cyamemazine, calculated from the IC50/receptor affinity ratios, is 81 and 313 against dopamine D2 receptors and 5-HT2A receptors, respectively. In guinea pigs, QT and QTcBazett were not significantly modified by intravenous cyamemazine when compared to the effects produced by the vehicle. Conversely, terfenadine (5 mg/kg iv) increased significantly QTcBazett (+58 ms), QTcFrediricia (+83 ms) and QTcVan de Water (+78 ms). In conclusion, cyamemazine concentrations required to inhibit hERG current exceed substantially those necessary to achieve therapeutic activity in humans. Moreover, cyamemazine, in contrast to terfenadine, does not delay cardiac repolarization in the anesthetized guinea pig. These non-clinical findings confirm the excellent cardiac safety records of cyamemazine during its 30 years of extensive therapeutic use.