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Intracellular pH (pHi), and Na+ and Cl- activities were measured with ion-sensitive micro-electrodes in Helix aspersa neurones, and the effects of reducing external pH (pHo) were investigated. When pHo was changed from 7.5 to 6.5 keeping CO2 constant, there was a slow fall in pHi, a rise in internal Cl and a fall in internal Na. These ionic changes are opposite to those caused by normal operation of the pHi-regulating system. These effects of external acidification were inhibited by the application of SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) or by the removal of external Cl. Raising intracellular Na activity by inhibiting the Na pump increased the rate of fall of pHi in acid solutions. In acid solutions the average rate of acid uptake attributable to reversed pHi-regulation was about three times the rate of loss of internal Na, and about twice the rate of Cl uptake. We conclude that these intracellular ion changes in acid solutions were largely due to a reversal of the pHi-regulating mechanism, so that it carried acid into, rather than out of, the cell interior.