Our understanding of the metabolism of chylomicrons, the lipoprotein that transports dietary fat from the intestine to peripheral tissues, is incomplete. The present studies were conducted to determine whether a labeled intravenous lipid emulsion could be used to estimate chylomicron triglyceride (TG) rate of appearance (R_a) and thereby quantify the rate of intestinal fat absorption. After an overnight fast, healthy volunteers (n = 6) sipped a ³H-labeled drink over 6.5 h at a rate of 175 mg fat · kg¹ · h¹. Beginning at hour 5, an HPLC-purified, ¹⁴C-labeled lipid emulsion was infused intravenously for 90 min. During the study, plasma total and chylomicron TG concentrations increased from 100 ± 21 to 237 ± 40 mg/dl and from undetectable to steady-state levels of 35 ± 13 mg/dl, respectively. After a minor correction for VLDL contamination, tracer-determined chylomicron TG R_a was 175 ± 30 mg · kg¹ · h¹, equal to the presumed ingestion rate. In summary, a radiolabeled intravenous lipid emulsion is able to accurately estimate chylomicron TG R_a and therefore can be used to measure in vivo fat absorption rates.