A new radioimmunoassay for secretin was used to investigate (a) serum secretin responses to intraduodenally infused HCl and glucose, (b) the metabolic half-life and the volume of distribution of exogenous secretin and (c) the effect of endogenously released secretin on insulin secretion in 25 anesthetized dogs. Portal and femoral venous blood samples were taken simultaneously before, during, and after intraduodenal infusion of HCl (21 meq/30 min) and glucose (131 ml/30 min). Control experiments were performed with intraduodenal infusion of saline.Mean portal venous immunoreactive secretin concentration of six dogs rose from 313 μU/ml before to 1,060 μU/ml 10 min after initiation of the intestinal acidification (P < 0.005). Femoral venous immunoreactive secretin concentration rose from 220 μU/ml before to 567 μU/ml 15 min after intestinal acidification (P < 0.01). Secretin concentrations remained elevated during the remainder of the infusion.In the same six dogs mean portal venous immunoreactive insulin concentration rose from 38 μU/ml before to 62 μU/ml at the end of the infusion (P < 0.05). Peripheral immunoreactive insulin, glucose, and free fatty acid concentrations, however, did not change significantly.Pancreatic exocrine function was studied in four dogs. The rise in secretin concentration was followed promptly by a highly significant increase in exocrine pancreatic flow rate and bicarbonate secretion, indicating biological activity of the circulating immunoreactive secretin.The effect of intraduodenal infusion of glucose on immunoreactive secretin concentration was studied in 12 dogs. Glucose in concentrations ranging from 2.5% to 10% had no detectable influence on portal or peripheral secretin concentration. Infusion of 50% glucose caused a slight decline in secretin concentration.The metabolic clearance rate, half-life of disappearance, and volume of distribution of exogenous secretin was studied in three dogs by the constant infusion technic. The metabolic clearance rate was 730±34 ml/min, volume of distribution was 17.4±0.8% of body weight, and the half-life of disappearance was 2.8±0.1 min. It could be calculated that 1.38 U/kg-h^-1 of endogenous secretin was released into the peripheral circulation during the steady state period of the HCl infusion experiments.The data indicated that immunoreactive secretin was released rapidly after intestinal acidification, continued to be secreted throughout the duration of HCl infusion, and was promptly distributed in the extracellular compartment. Furthermore, they suggested that endogenously released secretin could stimulate insulin secretion. The HCl-mediated insulinogenic effect of immunoreactive secretin, however, was too weak to influence peripheral immunoreactive insulin, glucose, and free fatty acid concentrations.The failure of intraduodenal glucose to stimulate secretin release suggests that secretin is not the insulin-stimulatory factor released from the gastrointestinal tract in response to glucose.