Several reports indicate that hypoglycemic sulfonylureas augment Ca²⁺-dependent insulin secretion via mechanisms other than inhibition of the ATP-sensitive K⁺ channel. The effect involves a 65-kd protein in the granule membrane and culminates in intragranular acidification. Lowering of granule pH is necessary for the insulin granule to gain release competence. Proton pumping into the granule is driven by a v-type H⁺-ATPase, but requires simultaneous Cl⁻ uptake into the granule via metabolically regulated ClC-3 Cl⁻ channels to maintain electroneutrality. Here we discuss the possibility that modulation of granule ClC-3 channels represents the mechanism whereby sulfonylureas directly potentiate the β-cell exocytotic machinery.