The small bowel transit time of 100 ml of lactulose solution infused at the ligament of Treitz was measured by breath hydrogen excretion in paired studies carried out in 43 healthy volunteers during infusion (1.2 ml/min) of equal volumes (100 ml) of isotonic solutions of either fat emulsion (Intralipid, Prosparol, or Calogen), protein hydrolysate, glucose, or saline into either the jejunum (90 cm from the teeth), ileum (205 cm from the teeth), or colon (350 or 400 cm from the teeth). Ileal infusion of Intralipid or protein hydrolysate resulted in significant delays in small bowel transit time (125 +/- 21 min and 71 +/- 11 min, respectively) compared with infusion of saline (50 +/- 3 min; p less than 0.02 and p less than 0.05). These delays were not associated with any significant alteration in plasma levels of neurotensin or enteroglucagon. Small bowel transit time was unaffected by infusion of nutrients into the colon or jejunum, although jejunal infusion of Intralipid increased the plasma levels of enteroglucagon and neurotensin (p less than 0.01 and p less than 0.02, respectively) after the start of lactulose infusion. In a separate series of paired experiments, infusion of Intralipid into the ileum in 5 volunteers significantly delayed the transit of a solid test meal labeled with 25 microCi of 99mTc-sulfur colloid through both the stomach and small intestine. These data support the existence of a mechanism whereby the presence of unabsorbed food in the ileum may enhance absorption by delaying the passage of food through the small intestine.