Potassium channels encoded by human Ether-à-go-go-Related Gene (hERG) underlie the cardiac rapid delayed rectifier K+ channel current (IKr), which plays an important role in ventricular action potential repolarisation (Sanguinetti and Tristani-Firouzi, 2006).Acidosis occurs in a number of pathological situations and there is consensus that extracellular acidosis can attenuate the amplitude and accelerate the deactivation of IhERG (Berube et al. 1999, Du et al. 2010). However, comparable data are lacking for intracellular acidosis. Replacement of extracellular sodium chloride (NaCl) with sodium acetate (NaAc) is established experimentally to produce intracellular acidosis, as uncharged protonated acetate can cross the cell membrane and then release protons intracellularly (Thomas 1984, Saegusa et al. 2011). Here, we investigated the effects of superfused 80 mM NaAc (Saegusa et al, 2011) or sodium butyrate (NaBu; which releases protons intracellularly similar to NaAc and so was used to determine whether or not observed results were acetate-specific) on hERG current (IhERG) stably expressed in Human Embryonic Kidney (HEK). Data are presented as mean ± standard error of the mean (SEM). Comparisons between control and test conditions were made using a paired t test, with significance denoted by p<0.05. Whole-cell patch-clamp measurements of hERG current (IhERG) were made at 37 oC. IhERG ‘tail’ amplitude at -40 mV, following a 2s depolarisation to +20 mV decreased rapidly when cells were superfused with 80 mM NaAc (by 25.6+3.1%; n= 6 cells) or NaBu (by 22.5+3.7%; n= 6 cells) solution (p<0.001 and p< 0.01 versus control for NaAc and NaBu respectively). IhERG tail deactivation rate also slowed with both interventions. Half-maximal activation voltage (V0.5 act) was shifted by-16.5+1.5 mV (n= 7 cells; p<0.001) and -17.3+1.6 mV (n=6 cells; p<0.001) respectively by NaAc and NaBu. The V0.5 of inactivation was negatively shifted by 80 mM NaAc (-7.9+1.3 mV; n=6 cells, p<0.01). These findings provide evidence that increasing the intracellular proton concentration produces multiple effects on IhERG, with modulation of both current amplitude and kinetics.