SUMMARY1. The secretion of sodium, potassium and lithium has been studied in the isolated cat pancreas, perfused with bicarbonate buffered saline solutions of varying composition and osmolality, and stimulated maximally with secretin.2. Under isosmolal conditions, when perfusate sodium chloride was replaced by sucrose, sodium secretion and potassium secretion were directly related to perfusate sodium concentration, [Na] The concentration of potassium in the secretion was almost identical with that in the perfusate over the whole concentration range.6. Replacement of perfusate sodium by lithium reduced the volume of secretion, though a small secretion was maintained even in the complete absence of sodium. The concentration of lithium in the secretion was generally slightly greater than that in the perfusate.7. Omission of potassium from the perfusate reduced secretion by about 65 %. Rubidium was a complete substitute for potassium; caesium was not.