The rapid delayed rectifier K+ current (IKr) plays a key role in ventricular repolarization in the mammalian heart (Witchel and Hancox, 2000). The channels carrying IKr are considered to be formed by subunits encoded by human ether-à-go-go-related gene (hERG); however there is recent evidence that native IKr channels may be comprised of hERG1a (the prevalent isoform) together with hERG 1b (a shorter isoform with a truncated N-terminus (Jones et al, 2004). Although acidosis occurs in a number of pathological situations and can modulate IKr and hERG current (IhERG) (e.g. Vereecke and Carmeliet, 2000; Berube et al, 1999; Du et al, 2010), the effects of acidosis on co-expressed hERG 1a/1b channels are unknown. Here, we investigated the effects of extracellular acidosis on hERG 1a/1b channels co-expressed (via transient transfection at a 1:1 ratio) in Chinese Hamster Ovary (CHO) cells. Whole cell patch clamp measurements of IhERG were made at 37 oC. IhERG was rapidly and reversibly decreased by reducing external pH from 7.4 to 6.3. Acidification of the extracellular solution rapidly reduced peak IhERG during ventricular action potential (AP) waveforms by 46.2 ± 1.9 % (mean ± S.E.M; n= 7 cells). Maximal IhERG conductance was also reduced (from 1.50 ± 0.2 nS/pF at pH 7.4 to 0.9 ± 0.1 nS/pF at pH 6.3; P < 0.001, paired t-test; n=10 cells). The voltage-dependence of activation was positively shifted by acidic solution by ~ 7 mV, but the time-course of activation was unaffected. Both fast and slow time constants of deactivation were smaller across a range of voltages at pH 6.3. Experiments with hERG1b expressed alone confirmed that acidosis could accelerate deactivation of this shortened N-terminus isoform. These findings indicate that the major effects of acidosis on IhERG with hERG1a/1b co-expression are qualitatively similar to those seen for hERG1a alone under identical recording conditions (Du et al, 2010) and support the notion that the contribution of IKr to ventricular action potential repolarization is likely to be reduced under acidic conditions.