In 39 patients the following were assessed: gastric acid secretion (mmol/hour), gastric juice volume (ml/hour), glycemia (mg%), and in 10 of them also pepsinogen I serum concentration (ng/ml). All these parameters were determined in basal conditions and after administration of glucagon. Glucagon induced achlorhydria in 33.3% of the patients, hypochlorhydria in 33.3%, did not influence HCl in 12.8%, and hyperchlorhydria in 20.5%. The gastric juice volume changed in parallel with chlorhydria. The HCl secretion did not correlate with the glycemia levels. Pepsinogen I was not significantly influenced by glucagon. It may be concluded that glucagon had a two-fold action on HCl secretion: (a) in two thirds of the patients studied it induced hypo- or achlorhydria; (b) in about 20% it induced hyperchlorhydria.