The chromaffin granule membrane in vitro is impermeable to protons as well as to Mg2+; however, when granules are incubated in the presence of the proton ionophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone or an inhibitor of the granule membrane Mg2+-dependent ATPase, the metal ion is accumulated inside the granules. This accumulation is dependent upon the granule transmembrane potential. The simultaneous presence of the ATPase inhibitor and the proton ionophore markedly increases metal ion incorporation. Mg2+ incorporation is also promoted by nigericin in the presence of potassium or sodium ions, indicating that Mg2+ accumulation is also dependent upon the transmembrane pH gradient. Concomitant with the Mg2+ accumulation, there is a significant loss of endogenous catecholamines. It is concluded that Mg2+ accumulation is determined by the electrochemical gradient maintained across the membrane. Once the metal ion has accumulated into the granules it displaces catecholamines from their storage sites.