The present study examines the suggestion that in the absence of adequate bile and pancreatic juice, which support the absorption from the gut of long-chain fatty acids into lymph, the fatty acids are absorbed directly into the portal blood. Oleic acid (18:1) partitioning between lymph and portal blood was investigated in intact and bile- and pancreatic juice-diverted rats. In a first set of experiments, 18:1 absorption from the gut into lymph and blood was studied by continuous recovery of the mesenteric lymph for 6 h and mesenteric portal venous blood for 1 h. In a second set of experiments, esterification processes were investigated by study of the mucosal distribution of labelled lipids and by mono- and diacylglycerol acyltransferase (EC 2.3.1.22 and EC 2.3.1.20 respectively) specific activities. In the bile- and pancreatic juice-diverted rats the absorption of labelled 18:1 into lymph was significantly reduced during the first 3 h of intraluminal infusion of this substrate. In such rats a compensatory absorption of labelled 18:1 into mesenteric portal blood was not observed. At 6 h after micellar lipid-mixture infusion, the overload of lipids both in free form and as triacylglycerols persisting in the mucosa paralleled the lower acyltransferase specific activities observed in bile- and pancreatic juice-diverted rats. These studies demonstrate the absence of a previously proposed compensatory absorption of 18:1 into portal blood when absorption into lymph is impaired by an inadequate supply of bile and pancreatic juice.