Dose-response studies of pentagastrin-stimulated gastric secretion were performed in 6 healthy volunteers. On different days pentagastrin was given in doses of 0.15, 1.5, and 15 mug/kg/hr either alone or in combination with carbacholine, 2 mug/kg/hr, or urecholine, 60 mug/kg/hr. Carbacholine and urecholine increased acid and pepsin secretion evoked by the lowest dose of pentagastrin while there was no augmentation at the highest dose. The dose of pentagastrin required to elicit half maximal acid output (Km) tended to decrease by simultaneous infusion of carbacholine or urecholine, suggesting that the cholinomimetics increased the sensitivity of the parietal cells to pentagastrin stimulation. Km for pentagastrin alone was higher than previously found in unoperated duodenal ulcer patients.