To test the regulatory effect of dietary triglyceride (TG) on rat lymphatic apolipoprotein B (apo B) transport, lymph-fistula rats were infused intraduodenally for 8 h at 3 ml/h with a lipid emulsion containing 40 mumol TG labeled with glycerol [9,10-3H(N)]triolein, 7.8 mumol egg phosphatidylcholine, and 57 mumol sodium taurocholate in phosphate-buffered saline with or without 1 mg/h Pluronic L81 (L81). L81 is known to prevent lipid transport in the intestine by inhibiting the formation of chylomicrons (CM). This action of L81 is quickly reversible by merely replacing L81 infusion by saline infusion. In the control rats (without L81 added to the infusate), the amount of apo B secreted in either whole lymph, CM, or the very-low-density lipoprotein (VLDL) fractions did not change significantly during lipid infusion compared with fasting. Compared with the fasting, the apo B output in lymph during L81 plus lipid or saline infusion in the experimental rats did not change significantly. The lymphatic apo B output data were also supported by the incorporation studies using [3H]leucine. In summary, these data demonstrate that the absorption of a physiological load of lipid into lymph does not affect the apo B synthesis in the mucosa or the secretion of apo B in lymph. Furthermore, the action of L81 is probably not by inhibiting intestinal apo B production because apo B secretion was not affected by the presence of L81. This study also demonstrates that the number of CM particles made by the small intestine remains relatively constant during fasting or active lipid uptake and transport. During active lipid absorption, instead of increasing the number of CM, the enterocytes expand the size of the CM particles. Lastly, the number and TG content of VLDL particles synthesized and secreted by the small intestine also seems to remain relatively constant during fasting and active lipid absorption.