The thiol reagent N-ethylmaleimide (NEM) completely inhibits the proton pump activity of the H+-ATPase in chromaffin granule 'ghosts' at concentrations which only partly (approximately 20%) inhibit the Mg2+-dependent ATP hydrolysis. Half-maximal inhibition was obtained at approximately 13 microM NEM as compared to 18 microM for the classical proton channel inhibitor N,N'-dicyclohexylcarbodiimide (DCCD), and the apparent stoichiometry of the inhibitors at complete inhibition was NEM : DCCD congruent to 1 : 2. HIgh concentrations of NEM (greater than 100 microM) induce a dissipation of the transmembrane potential generated by MgATP. These findings establish NEM as a valuable proton channel inhibitor in chromaffin granules and explain the rather complex effect of NEM previously reported for catecholamine accumulation in this organelle.