The mean fasting serum secretin level determined in newborn swine was found to be 298.0 +/- (SEM) 5 pg/ml, significantly higher than that in the adult animal. Intraduodenal infusion of 0.1 N HCl produced dramatic elevation of the mean serum secretin level to 2,090 +/- 340 pg/ml. The tissue secretin concentration of duodenal mucosa as well as the molecular species of tissue secretin were found to be identical to that of the adult swine. The mean disappearance half-life of exogenously administered secretin in the newborn swine was significantly prolonged over adult values, to 3.6 min. These data suggest that delayed secretin degradation can be implicated as a factor in the etiology of the hypersecretinemia in the newborn swine.