Anion replacement experiments were performed on superfused in vitro mouse pancreatic tissue and the effects on the electrical response of acinar cells to ACh investigated. 2. Electrical measurements were made with two micro-electrodes inserted into electrically coupled cells. ACh was applied by microionophoresis. Potential recordings were taken before, during and after changeover from the control superfusion fluid, containing Cl-, to one containing the substituted anion. 3. From the results obtained the tested anions were classified into three groups: I, Cl(-)-like anions: Br-, I- and NO3-, causing either no change or a negative displacement of the ACh null-potential, compared to that measured in the control Cl(-)-containing solution, and only small changes in the resting and stimulated electrical properties of the acinar cell, II, ions less permeable than Cl-: isethionate, acetate, sulphate and hippurate, showing a positive displacement of the ACh null-potential and a similar or increased resting cell input resistance, and III, methylsulphate and benzenesulphonate, causing a negatively displaced ACh null-potential but showing changes in the resting electrical properties of the acinar cells characteristic of anions in group II. 4. The ACh null-potential sequence, in order of decreasing negativity, was NO3- greater than or equal to benzenesulphonate greater than or equal to I- greater than or equal to methylsulphate greater than Br- greater than or equal to Cl- greater than isethionate greater than acetate greater than or equal to sulphate greater than hippurate. 5. Experiments involving the use of bicarbonate demonstrated that it does not contribute significantly to the value of the ACh null-potential. 6. The sequence of the anions in group I were compared to the Eisenman series I, suggesting that the ACh-opened Cl- pathway comprises a large hydrated ion channel bearing a lining of weak positive charges. 7. A quantitative relationship was sought between the ACh null-potential and extracellular Cl-. It was found that a tenfold reduction in the extracellular concentration resulted in a 15 mV positive shift of the null-potential.
