Previous studies [C. M. Mansbach II and A. Arnold. Am. J. Physiol. 251 (Gastrointest. Liver Physiol. 14): G263-G269, 1986] have shown that only 41% of the mass and 54% of the disintegrations per minute (dpm) are recovered in lymph on duodenal infusion of 3H-labeled glyceryl trioleate (TO). The present studies evaluate the potential that the unaccounted for lipids were transported via the protal vein. Rats received portal vein, carotid artery (surrogate for the mesenteric artery), and duodenal cannulas and an ultrasonic flow probe around the portal vein. [3H]TO (135 mumol/h) was infused into the duodenum, and blood samples were collected from the other cannulas. Portal flow was recorded. Portal fatty acid (FA) increased from 220 to 705 nmol/ml during the 6-h infusion and was 201 mumol/h in excess in the portal vein vs. the carotid artery. All common FAs were increased in the portal vein vs. the carotid artery except C18:3 and C20:4. Thirty-nine percent of the dpm infused was calculated to be transported via the portal vein. We conclude that considerable endogenous FA and infused lipid, mostly as triacylglycerol, are transported in the portal vein in response to TO infusion. We speculate that TO infusion induces endogenous FA mobilization from the mesenteric bed.