The endogenous release of secretin in healthy human subjects was studied by measuring plasma secretin levels by radioimmunoassay (RIA) before, during, and following duodenal acidification and eating a meal. The sensitivity of our RIA was assessed by measuring plasma secretin levels during constant intravenous infusions of 4 graded doses of secretin. Our RIA detected significant increases (P<0.001) in plasma secretin with each dose, including a low dose of 0.125 U (41.3 ng)/kg hr. Intraduodenal infusion of 0.1N HCl resulted in marked increases (P<0.001) in plasma secretin levels whenever pH in the second portion of the duodenum was reduced to less than 3.5. In contrast, following a meal, pH in the second portion of the duodenum remained consistently greater than 4.5 and plasma secretin levels showed no changes from basal levels. These studies confirm that endogenous release of secretin depends on an acidic pH of the duodenum, and insignificant changes in the plasma secretin level following ingestion of a meal suggest that endogenous release of secretin in the postprandial period is probably too small in quantity to be detected by the present radioimmunoassay method.