Stimulation of the exocrine pancreas is one of the biological effects attributed to the tridecapeptide neurotensin (NT). Only the C-terminal 8-13 fragment with its extremely short plasma half-life in the peripheral circulation was considered to be necessary for biological activity. The N-terminal fragments that are detectable in peripheral blood in much greater concentrations were considered to be of no importance. In order to elucidate the biological relevance of these N-terminal fragments, five dogs were infused with neurotensin at a rate (50 pmol/kg/h) that had been determined to give physiological plasma concentrations. Pancreatic secretions were collected from previously implanted modified Herrera pouches. The effects of the N-terminal fragments NT 1-8 and NT 1-11 on the exocrine pancreas secretion were tested in the same way. It was possible for the first time to demonstrate a biological effect of the N-terminal fragments found in the postprandial circulation in vivo. The plasma concentrations determined by radioimmunoassay remained within the physiological range (less than 40 pg/ml). These N-terminal neurotensin fragments may therefore participate in the hormonal regulation of the exocrine pancreas secretion.