Gastrin Leo was injected subcutaneously (2 μg/kg body weight) in 10 patients on 19 occasions, and the gastric juice was aspirated continuously for 1 hour before and 1 hour after the injection. The gastric juice was divided into 20-minute portions and analysed for volume and content of TA, K, Na, Mg, Ca, and P. Concentrations ([ ]) of TA, K, and Mg increased after injection of gastrin, while [Na], [Ca] and [P] decreased. The outputs (Q) of TA, K, and Mg increased significantly, while the QNa and QP showed no significant changes and the QCa decreased after stimulation with gastrin. Statistically, there were highly significant negative correlations between [TA] and [Ca] and between [TA] and [P], Significant positive correlations existed between [Ca] and [P] and between [Ca] and [Mg], There existed a statistically significant correlation between QTA and QMg, and between OTA and QCa, and also between QMg and OK, and QCa. The QCa showed significant relations to all parameters, while the QP was only statistically significantly correlated to QCa. A study of the reproducibility of concentrations and outputs of these ions before and after the injection of gastrin showed that there was a lower grade of reproducibility of Mg, Ca and P than that observed for TA, K, Na, and Cl in the same patients. The values obtained in this study are similar to those found in studies utilizing other secretogogues, mainly histamine. However, the secretion of magnesium was in some respects different from that found earlier, and the conclusion was drawn that this ion stems from other non-parietal sources than do Ca and P.