The effect of the gastric alkaline tide on serum-ionized calcium levels was determined in human subjects. Gastric acid seretion was stimulated by a standard steak meal, human synthetic gastrin, and betazole hydrochloride. Ionized calcium levels fell to a similar extent after each stimulus. The mean decrease in calcium ion concentration in all experiments was 5.4 % of the basal concentration. The fall in serum calcium ion concentration was highly correlated with the rise in serum pH. We speculate that increased formation of calcium bicarbonate complex in the serum as well as increased binding of ionized calcium by serum protein accounts for the surprisingly large effect of the alkaline tide on serum-ionized calcium levels.