The production of 14CO2 from uniformly labeled glucose was shown to account for the entire increase in histamine-stimulated O2 consumption in rabbit gastric glands when no other substrate was added to the medium. The increased production of CO2 was correlated to the increase in O2 consumption and the accumulation of [14C]-aminopyrine (AP) after stimulation with several secretagogues. Inhibitors of H(+)-K(+)-ATPase reduced the secretagogue-induced increase in CO2 production by greater than 90%, showing that the activity of this enzyme was responsible for the greater part of gastric gland metabolism under stimulated conditions. In contrast to AP accumulation, inhibition of CO2 production by omeprazole, an acid-activated inhibitor of the H(+)-K(+)-ATPase, was not reversed by washing. The reversal of AP accumulation after omeprazole treatment and washing was most likely due to a recruitment of residual pumps bordering a nonacidic space, which had not previously been inhibited by omeprazole. These residual pumps slowly generate a pH gradient and hence AP uptake. Adding NH4+ to gastric glands resulted in a concentration-dependent increase of CO2 production up to the maximal stimulated level but without formation of the pH gradient as measured by AP uptake and loss of the omeprazole inhibition of glucose oxidation. As NH4+ can act as a K+ surrogate for H(+)-K(+)-ATPase, and as NH3 is membrane permeant, full stimulation of CO2 production is evidence that the major mechanism of H(+)-K(+)-ATPase activation in situ is an increase in the KCl permeability of the pump membrane.