The short QT syndrome (SQTS) is a recently discovered condition characterized by abbreviated ventricular repolarisation and an increased risk of cardiac arrhythmia (McPate et al, 2006). One variant of the syndrome has been reported to result from a gain-of-function mutation (D172N) to the KCNJ2 gene which results in increased outward current through Kir 2.1 K⁺ channels at ambient temperature (Priori et al, 2005), possibly because the mutation affects channel block by intracellular Mg²⁺ and polyamines (e.g. Stanfield et al, 1994; Matsuda et al, 2003). Here the effects of the D172N mutation on whole-cell Kir 2.1 current were investigated at 37°C using voltage ramp and cardiac ventricular action potential (AP) waveforms. Wild-type (WT) and D172N Kir2.1 channels were transiently expressed in Chinese Hamster Ovary cells and whole-cell recordings made using a standard external Tyrode’s solution and K⁺-based pipette solution. Average data are expressed as mean ± S.E.M values and statistical comparisons utilised an unpaired t test. For each of WT and D172N Kir2.1 (n = 8 and 23 cells respectively), current during ascending voltage-ramps (velocity of 0.16 V s^-1) was normalised to maximal inward current at -120 mV (current at 120 mV was given a normalised value of -1; Priori et al, 2005). The profile of normalised inward current was similar between WT and D172N Kir2.1 expressing cells and the measured reversal potential was similar (-86.8 ± 1.0 mV and -86.3 ± 0.7 mV respectively; n= 8 and 23; p>0.05). Outward current was augmented for D172N compared to WT Kir2.1 (normalised maximal values of +0.25 ± 0.04 versus +0.13 ± 0.03 p<0.05) and occurred at similar voltages (-64.2 ± 1.6 mV versus -66.2 ± 1.6 mV p>0.05). A digitised ventricular AP was also used to stimulate D172N and WT Kir2.1 expressing cells and current sensitive to rapid application of 1mM Ba²⁺ was measured. In each of eleven D172N Kir.2.1 expressing cells, pronounced Ba²⁺-sensitive current was seen during AP repolarisation, whereas in four experiments on WT Kir2.1 Ba²⁺-sensitive current during repolarisation was less pronounced. Instantaneous current-voltage plots for these experiments showed a positive voltage-shift in maximal current during AP repolarisation for D172N compared to WT Kir 2.1 (maximal outward current occurring at 60.2 ± 2.5 mV and -72.4 ± 2.7 mV respectively; p<0.05). The findings of this study are consistent with a scheme in which Kir2.1 contributes augmented outward current during the later stages of ventricular repolarisation, leading to abbreviated repolarisation in variant 3 SQTS (Priori et al, 2005).