The steady state rate of ATP hydrolysis (v) by the gastric H,K-ATPase and the steady state level of phosphoenzyme (E-P) have been measured at 0 and 10 mM KCl; both v and E-P have a nonhyperbolic dependence on the ATP concentration that is consistent with negative cooperativity. The ratio of the rate of hydrolysis to phosphoenzyme (v/[E-P]) was found to vary with the concentration of ATP. Thus, for the rate law v = [E-P].k, k must be a function of the ATP concentration. This requires that ATP be able to bind to E-P or to an enzyme form that occurs after E-P but prior to an irreversible step, such as the loss of inorganic phosphate (Pi). At low ATP concentrations, product inhibition by Pi gives concave downward plots of 1/v against Pi concentration. Pi increases the apparent Km and decreases the apparent Vm. At saturating ATP concentrations, Pi is a noncompetitive inhibitor. These data show that ATP and Pi can bind to the H,K-ATPase simultaneously. They are inconsistent with mechanisms where the binding of ATP and Pi is mutually exclusive.