Increasing attention is being focused on the relationship between gastrointestinal hormones and calcium metabolism. While it is apparent that some relationship does exist, the exact nature and significance is as yet undefined. Information regarding the effect of secretin on serum calcium has been both sparse and contradictory. Boot's secretin, 3 units/kg, was given to a group of 44 patients consisting of normal controls and patients with duodenal, gastric, and marginal ulcers; primary hyperparathyroidism; and pernicious anemia. Six serial serum specimens were obtained from each patient at ten-minute intervals for calcium determination by atomic absorption spectrophotometry. Using two-way analysis of variance, no significant changes in postsecretin serum calcium values were observed. It is unlikely that physiologic levels of secretin exert any effect on serum calcium.