In rats with intestinal lymph-fistula and electrolyte and protein losses continuously replaced by IV infusion, the plasma to lymph filtration of total protein, albumin, and cholesterol was similar after duodenal infusion of either long-chain (LCT) or medium-chain (MCT) triglyceride. Filtration of cholesterol into intestinal lacteals was also measured after pulsed IV administration of 14C-beta-sitosterol as an indirect marker for passage of lipoproteins into the lymph system. During intraduodenal administration of LCT and constant IV infusion of 125I-apo high density lipoprotein (HDL), intact plasma HDL appeared in lymph and contributed apo-LP to chylomicron formation. Nevertheless, most lymph apo-LP originated from local mucosal synthesis.