The cardiac rapid delayed rectifier potassium channel passes ionic current (IKr) that plays an important role in cardiac atrial and ventricular action potential (AP) repolarisation; recombinant channels encoded by human ether-à-go-go-related gene (hERG) pass current (IhERG ) that is similar to native IKr (Sanguinetti and Tristani-Firouzi, 2006). N-desethyl-amiodarone (DEA) is the major active metabolite of the widely used Class III antiarrhythmic agent amiodarone (Kodama et al, 1997). At present there is little information available as to the extent to which DEA is able to inhibit IhERG (Waldhauser et al, 2008). The aim of this study was to determine and characterise effects of DEA on IhERG. Whole cell patch-clamp recordings were made at 37oC from Human Embryonic Kidney (HEK 293) cells stably expressing hERG. DEA progressively inhibited IhERG over 10 minutes of drug exposure. DEA inhibited IhERG with a half-maximal inhibitory (IC50) value of 157.6 ± 21.2 nM, compared to 47.0 ± 5.0 nM for amiodarone (n=5 replicates for each of 5 DEA and 5 amiodarone concentrations tested). Analysis of current-voltage (I-V) relations for IhERG ‘tails’ in control and in the presence of 500 nM DEA showed significant voltage-dependence of IhERG inhibition (P<0.0001 for fractional block over the range -30mV to +40mV; one-way ANOVA, n=6 at each voltage). DEA also induced a -9 mV leftward shift in voltage-dependent activation of IhERG. IhERG blockade by DEA was also observed to increase progressively with duration of applied depolarising voltage command. A 3-step voltage command (a single command comprised of depolarisation from -80 to 0 mV; 0 to +40 mV and then +40 to 0 mV) was employed to investigate open and inactivated channel blockade. There was no significant change to the fractional block by DEA of IhERG during the step to +40mV (to increase extent of IhERG inactivation; P> 0.1; ANOVA, n=10), this suggests that DEA did not exhibit a strong preference for activated over inactivated hERG channels, or vice versa. Collectively, these findings indicate that DEA blocks IhERG with a sub-micromolar IC50 value and that it can interact with both activated and inactivated hERG channels.