The hERG channel mediates the IKr potassium current, which regulates ventricular action potential duration and, thereby, the duration of the QT interval. HERG is a target for Class Ia and III antiarrhythmic drugs as well as for non-cardiac drugs associated with drug-induced Long QT Syndrome. Prior work has established that the Class Ia agent disopyramide (DISO) inhibits hERG current (IhERG) significantly at clinically relevant concentrations by binding within the channel’s inner cavity (Paul et al, 2001; El Harchi et al, 2012). DISO exists as a racemic mixture of (S+) DISO and (R-) DISO. However, whilst stereoselectivity has been established for some hERG-blocking compounds (Lin et al, 2009; Grilo and Abriel, 2010), little is known for DISO in this regard. In this study, we have investigated the potential for stereoselective IhERG inhibition by DISO.Whole-cell patch-clamp measurements of IhERG were made at 37°C from hERG-expressing HEK293 cells. All observations reported here are from at least 5 replicates. With a conventional voltage-step protocol, observing IhERG tails at -40 mV following depolarisation to +20 mV, (S+)DISO was found to block wild-type IhERG ~3 -fold more potently than did (R-)DISO (with respective IC50 values of 3.9±0.6 μM and 12.9±1.4 μM). With high (94 mM) [K+]e, the IC50 values for (S+) and (R-)DISO were 16.9 ±3.0 μM and 61.1±7.4 μM respectively. The F656A mutant of the key S6 binding residue for DISO reduced the potency of IhERG by both DISO enantiomers, with a ratio of F656A/ wild-type IC50 of ~29 for (S+)DISO and ~ 20 for (R-)DISO. Our data provide evidence for stereoselectivity of IhERG block by DISO: (S+) DISO is more potent against IhERG than is (R-) DISO. This may correlate with a slightly stronger interaction with the F656 aromatic residue for (S+) than for (R-) DISO.