We examined the mesenteric lymphatic and portal venous transport of triglyceride (TG), free fatty acids (FFA), apolipoproteins (apo)A-IV and B in response to a bolus duodenal infusion of a TG-free control solution, and long-chain (18:1) and medium-chain (8:0) TG (LCT and MCT, respectively) emulsions in the rat. Additionally, intestinal and hepatic apo A-IV and apo B mRNA levels were also measured. Lymph apo A-IV, apo B (B-48), FFA, and TG output increased after LCT infusion, whereas only apo A-IV and FFA outputs increased after MCT infusion. On the other hand, portal FFA and apo A-IV transports increased at 15 min after MCT infusion but not after LCT infusion. Portal TG and apo B transports were not altered by either MCT or LCT infusion. Jejunal apo A-IV mRNA was increased after both MCT and LCT infusions. Hepatic apo A-IV mRNA levels increased only after MCT infusion. Conversely, neither LCT nor MCT had any effect on apo B mRNA levels in intestine or liver. These results indicate that apo A-IV is regulated by MCT absorption and that fatty acid reesterification and lipoprotein assembly are not prerequisite for such regulation. Conversely, it is likely that apo B-48 participates only in the formation and/or transport of chylomicrons after LCT absorption.