1. In the isolated cat pancreas, stimulated maximally with secretin, increasing the perfusate potassium concentration (at the expense of sodium ions) caused a copious secretion of amylase from the gland, reduced the volume rate of secretion and caused vasoconstriction.2. Rubidium and caesium had similar effects to potassium: lithium, though depressing secretory rate, had no effect on enzyme secretion or vasoconstrictor action.3. Amylase secretion was detected at potassium concentrations of 30 mM and was maximal at 80-90 mM, output declining as the concentration was raised to 120 mM.4. Amylase secretion was maximal during the first few minutes of exposure to excess potassium, but remained above basal levels throughout the test period. Secretory rate was depressed by a constant amount during the test period.5. Atropine sulphate blocked the effect on enzyme secretion without affecting the reduction in secretory rate.6. During perfusion with excess potassium a vasodepressor material with the properties of acetylcholine was detected in the effluent from the gland.7. The reduction in secretory rate, when perfusate sodium was replaced by potassium, was equal to that obtained when sodium was replaced by sucrose.8. It is concluded that potassium stimulates amylase secretion indirectly by releasing acetylcholine from nerve terminals in the gland, and that the reduction in secretory rate is due not to excess potassium but to sodium deficiency.