Measurements of the effects of NH 3 /NH 4 + on glucose-induced electrical activity in β-cells from microdis-sected mouse islets of Langerhans and on intracellular pH in single collagenase-isolated islets pre-loaded with a fluorescent pH probe were performed and are reported here. Application of NH 3 /NH + 4 (15 mM) in the presence of glucose (11 mM) promptly hyperpolarized the β-cell membrane, reduced input resistance by 60% and blocked electrical activity. These changes were paralleled by an increase in islet fluorescence indicative of a cytosolic pH increase. Removal of NH 4 Cl initially stumulated electical activity, which returned to resting level with a time constant of 51 s. Concomitant with the removal of NH 4 C1 there was a drop in pH i followed by a slow return to resting level with a time constant of 83 s. The results suggest that the [Ca 2 + ]-dependent K + channel in the β-cell membrane is activated by a rise in cytosolic pH.