The membrane potential (ΔΨ) and the pH gradient (ΔpH) across the membrane of the insulin-secretory granule were determined in studies in vitro from the uptake of the permeant anion thio[¹⁴C]cyanate or the permeant base [¹⁴C]methylamine. Freshly prepared granules incubated in iso-osmotic medium containing sucrose and low concentrations of buffer salts exhibited an acidic internal pH and a ΔΨ positive inside. Addition of MgATP²⁻ under these conditions did not alter the ΔpH, but produced a marked increase in the ΔΨ. Conversely, when a permeant anion was also included, ATP produced a marked increase in the ΔpH and a lesser increment in the ΔΨ. NH₄⁺ salts reduced the ΔpH across granule membranes. In the presence of ATP this effect was accompanied by a reciprocal increase in the ΔΨ. A similar reciprocity was evident when nigericin was added together with K⁺ or on decreasing the medium pH, suggesting that these gradients were linked by a common electrogenic process. The effects of ATP were reversed by the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, the combination of valinomycin, nigericin and K⁺, and by the Mg²⁺-dependent ATPase inhibitor tributyltin. Uptakes of ¹⁴C-labelled tracer molecules were also markedly reduced by cryogenic disruption of the granule membrane or hypo-osmotic incubation conditions. These results were readily interpreted within a chemiosmotic hypothesis, which proposed that the insulin granules possess an inwardly-directed electrogenic proton-translocating Mg²⁺-dependent ATPase with the additional postulate that the membrane has a low proton permeability. The intragranular pH was estimated as being between 5 and 6 in vivo. Such a value corresponds to optimal conditions for the crystallization of zinc–insulin hexamers. Several other functions related to chemiosmotic processes within insulin granules, however, may be envisaged.