It is well-established that the neuronal plasma membrane Ca-ATPase carries H+ ions into the cell at the same time as it pumps out Ca2+ ions (Schwiening et al, 1993, see also Niggli & Sigal 2008). This results in pH changes both inside and outside cells. I have recently reported that the coupling ratio in snail neurones at pHo 7.5 is close to 1Ca:2H, in agreement with much but not all earlier work (Thomas, 2009). The PMCA Ca:H coupling ratio in erythrocytes and barnacle muscle has been reported to vary with pHo from about 1:2 at pH 6.5 to 1:0 at pH 8.5 (Milanick, 1990) or from 1:3 at pH 6.5 to 1:1 at pH 8.2 (DeSantiago et al 2007). I have now, using the same methods as previously reported, measured the effects of increasing or decreasing pHo by one unit on the coupling ratio in snail neurones. I injected Ca2+ iontophoretically and measured the resulting intracellular pH (pHi) changes at pHo values of 6.5, 7.5 and 8.5, as shown in Fig 1. In this representative experiment each injection caused a fall in pHi which was little different when the external pH was increased or decreased by one pH unit. In a total of 5 similar experiments, in some of which I also measured intracellular Ca2+ changes, I made 29 injections at the control pHo 7.5, 17 injections at pH 8.5 and 15 at pH 6.5. I found that the average fall in pHi per unit charge injected at pH 8.5 was 0.93 ± 0.06 (S.E.M., n = 5) of that for the control pH, while at pH 6.5 the average was 1.04 ± 0.06 of control. These findings are very different to those previously reported for red blood cells and barnacle muscle.