The mechanism of histamine and cholinergic stimulation of parietal cell acid secretion was investigated in isolated rabbit gastric glands. The binding of a histamine antagonist to an H2 receptor had a KD of 1.2 μM with the number of sites per cell being about 5000. The binding reaction activated adenylate cyclase and the action of various histamine analogs on acid secretion or changes in O2 consumption depended on changes of cAMP or cyclase levels. cAMP bound exclusively to the regulatory subunits of type I and II protein phosphokinases as assessed by N3-cAMP binding. Cholinergic stimulation depended on medium Ca++, and the role of calmodulin was shown by the inhibition of acid secretion in intact glands by trifluoperazine or chlorpromazine with a KD of 0.5 - 1 μM. Trifluoperazine also inhibited ATP dependent acid secretion in digitonin permeabilized glands with a similar KD. Calmodulin-like activity was associated with gastric vesicles. A model is presented for interaction of Ca++ and cAMP on acid secretion, involving pump activation and enhancement of Cl permeability to allow KCl flux across apical membranes. © 1982.