We measured the plasma concentration of neurotensin-like immunoreactivity (p-NTLI) after administration of Intralipid (mainly triglycerides), or oleic acid perorally, into the intact proximal jejunum, intact distal jejunum or proximal ileum and together with partly digested Intralipid into an isolated ileal loop in man. Administration per os or into the proximal jejunum gave the highest p-NTLI responses, administration into the intact distal jejunum or proximal ileum gave a lower response and administration into the isolated ileal loop gave no response at all, although approximately 25 percent of the neurotensin producing cells should be located there. As estimation of intestinal tissue content of NTLI showed that in small intestine which for a long time had been out of contact with food there was no sign of depletion of NTLI, we suggest that nervous, humoral or luminal factors provoked by fat in the upper gastrointestinal tract must trigger the N-cells before NTLI can be released by fat exposition.