The initial rate of ATP-dependent proton uptake by hog gastric vesicles was measured at pH's between 6.1 and 6.9 by measuring the loss of protons from the external space with a glass electrode. The apparent rates of proton loss were corrected for scalar proton production due to ATP hydrolysis. For vesicles in 150 mM KCl and pH 6.1, corrected rates of proton uptake and ATP hydrolysis were 639 +/- 84 and 619 +/- 65 nmol/min x mg protein, respectively, giving an H+/ATP ratio of 1.03 +/- .07. Furthermore, at all pH's tested the ratio of the rate of proton uptake to the rate of ATP hydrolysis was not significantly different than 1.0. No proton uptake (less than 10 nmol/min x mg protein) was exhibited by vesicles in 159 mM NaCl at pH 6.1 despite ATP hydrolysis of 187 +/- 46 nmol/min x mg (nonproductive hydrolysis). Comparison of the rates of proton transport and ATP hydrolysis in various mixture of KCl and NaCl showed that the H+/ATP stoichiometries were not significantly different than 1.0 at all concentrations of K+ greater than 10 mM. This fact suggests that the nonproductive rate is vanishingly small at these concentrations, implying that the measured H+/ATP stoichiometry is equal to the enzymatic stoichiometry. This result shows that the isolated gastric (K+ + H+)-ATPase is thermodynamically capable of forming the observed proton gradient of the stomach.