As calcium is important in the regulation of gastric acid secretion and gastrin release, we have examined the effect of the calcium antagonist nifedipine on these processes in man. Nifedipine 30 mg orally inhibited basal acid output by 37% (p less than 0.025) and that stimulated by low infusion rates of pentagastrin--that is, 0.031 and 0.062 microgram/kg/h by 44% (p = 0.05) and 39% (p less than 0.02) respectively. On increasing the pentagastrin infusion rate the inhibition was surmounted suggesting it was competitive in type. Nifedipine did not affect basal or Oxo meal stimulated gastrin concentrations in normal volunteers nor did it affect resting serum gastrin or calcium stimulated increase in gastrin in a single patient with Zollinger-Ellison syndrome. These findings are consistent with the transmembrane flux of calcium ions being involved in basal and pentagastrin stimulated acid secretion in man.