Abstract The relation between fractional catabolic rate (K 2) of an intravenously injected fat emulsion, Intralipid ®, and the level of serum triglyceride (TG) was evaluated to cast light on TG-rich lipoprotein metabolism in 182 subjects who were homozygotes for the most common form of apoE 3 3 . Both normolipidemic individuals and primary hyperlipidemic patients were included. To assess the influence of variation in the apoE phenotype on fat emulsion metabolism, 25 subjects with the apoE 4 3 phenotype and 21 with apoE 3 2 phenotype were also evaluated. In the apoE 3 3 subjects, K 2 decreased with increasing TG level up to 180mg dl , but above 180mg dl , K 2 remained at a constant level. This TG value was therefore concluded to be a cut off beyond which the TG-dependent decrease in K 2 disappeared. No aparent correlation between K 2 and serum TG was observed in subjects with apoE 4 3 or 3 2 phenotypes. In subjects with TG above 180mg dl , presumably both accelerated synthesis and limited removal are involved in the development of their hypertriglyceridemia. Since K 2 demonstrated little change with increase of TG over 180mg dl in apoE 3 3 subjects, we concluded that the capacity to catabolize fat emulsion reaches a kinetic saturation. The TG value of 180mg dl may be a physiological significance. The relation between K 2 and TG was specific in apoE 3 3 , as it was not observed in subjects having apoE 4 3 or 3 2 phenotypes.