We studied the route of transport of long-chain fatty acids (LCFA) from the rat intestine. Lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), linoleic (18:2), and linolenic (18:3) acids were infused intraduodenally for 4 h as micellar solutions into unanesthetized thoracic duct-fistula rats. Proportionally more of each LCFA was transported by the portal vein at infusion rates of 0.3 mumol/h than at 15 mumol/h. Of the LCFA absorbed at low rates of infusion, 72% of lauric, 58% of myristic, 41% of palmitic, 28% of stearic, 58% of linoleic, and 68% of linolenic acid bypassed the lymphatic pathway. To test the inference that 58% of absorbed linoleate was transported by the portal vein, [14C]linoleic acid was infused at 0.3 or 15 mumol/h into unanesthetized rats equipped with both thoracic duct and portal venous fistulas. Portal venous blood (0.1 ml) was withdrawn every 30 min for 4 h. Proportionally more [14C]linoleate was recovered in portal blood at the low infusion rate. To examine the morphology of fat absorption, segments of rat jejunum in anesthetized rats were infused with micellar linoleic acid at 3 or 150 mumol/h. At 3-mumol/h infusion rate, an appearance identical to the fasting state was seen by electron microscopy. At 150 mumol/h, many larger chylomicron-sized particles appeared in absorptive cells, intercellular spaces, and lymphatics. We conclude that a substantial proportion of unsaturated LCFA is transported from rat intestine in portal venous blood.