Four types of receptors have so far been identified on the parietal cell: receptors for gastrin, acetylcholine, and histamine, whose activation leads to stimulation of acid secretion, and receptors for prostaglandins of the E series, whose activation leads to inhibition of acid secretion. Postreceptor events following drug-receptor interaction include mobilization of calcium ions and cAMP formation. Moreover, a number of putative receptors (for somatostatin, CCK, adenosine, secretin, etc.) have been hypothesized, even though definite evidence is still lacking. Finally, other substances may influence gastric acid secretion in a positive or negative way, but no evidence for an involvement of specific receptors on the parietal cells is available so far.