The concentration of neurotensin-like immunoreactivity in plasma (p-NTLI) increases after the ingestion of food, and fat seems to be the most important nutrient. It is essential to characterize the NT species that are responsible for this postprandial rise of p-NTLI. After an overnight fast, two male and two female subjects therefore ingested 300 ml of cream (containing 40% (w/w) milk fat). Unextracted plasma samples were subjected to column chromatography and the eluates were analysed using four NT antisera having different specificities. The concentration of chromatographically identified NT(1-13) in peripheral plasma increased significantly from 3 pM in the fasting state to 26 pM 30 min after the ingestion of fat. The concentration of NT(1-8), which is probably a metabolite of NT(1-13), also increased markedly. No significant increase of smaller COOH-terminal sequences of NT was found. The results show that the plasma concentration of NT(1-13) may increase about tenfold following the ingestion of fat. This is further support for the hypothesis that NT(1-13) may function as a hormone.