Ingestion of fat causes a pronounced decrease in lower esophageal sphincter (LES) pressure. This may be due in part to effects of neurotensin (NT), which is released postprandially after the ingestion of fat. The aim of the present investigation was to establish whether the LES pressure decreases at physiological plasma concentrations of NT(1-13). In addition, we have compared the effects and metabolism of NT(1-13) and (Gln4)NT(1-13). The experiments were performed in 3 healthy male volunteers who had fasted for at least 11 hours prior to the study. NT(1-13) or (Gln4)NT(1-13) were infused intravenously at a dose of 12 pmoles X kg-1 X min-1. LES pressure was monitored by a continuous pull-through method with a perfused catheter. The concentrations of chromatographically identified tridecapeptides were determined using NH2 and COOH-terminal directed antisera. Already one min after the start of the infusions the LES pressure had decreased by 43% of the control value. At that time the plasma concentration of the neurotensin tridecapeptide was about 15 pM. NT(1-13) and (Gln4)NT(1-13) had similar half-lives in plasma (2.5 min). NT(1-8) and (Gln4)NT(1-8) were found to be the main metabolites of NT(1-13) and (Gln4)NT(1-13) respectively. The results indicate that the increase in the plasma concentration of NT(1-13) seen after the ingestion of food is sufficient for neurotensin to function as a hormone of the endocrine type. NT(1-13) and (Gln4)NT(1-13) have the same effects on LES pressure in humans and show the same pharmacokinetic characteristics.