The in vivo metabolism of esterified cholesterol and triglyceride in plasma very low density lipoproteins (VLDL) has been studied in postabsorptive rabbits injected with endogenously 3H-labelled preparations of VLDL. The rates at which the injected esterified [3H]cholesterol and [3H]triglyceride were removed from the recipient VLDL fraction were remarkably similar. But, whereas most of the [3H]triglyceride was rapidly lost from the plasma compartment, more than 60% of the esterified [3H]cholesterol removed from VLDL was recovered in the plasma low density lipoprotein (LDL) and high density lipoprotein (HDL) fractions, an observation in direct contrast to previous findings in rats and guinea pigs in which hepatic uptake accounted for the major proportion of esterified cholesterol removed from VLDL. The appearance of VLDL-esterified [3H]cholesterol in rabbit LDL was quite compatible with the well-documented catabolic conversion of VLDL to LDL. The appearance in HDL, on the other hand, could not be so explained and was further investigated in studies performed in vitro. It was found that, concurrent with a net mass transfer of esterified cholesterol from HDL to VLDL, there was a much more rapid transfer of esterified [3H]cholesterol in the reverse direction, implying the existence of a process of molecular exchange of esterified cholesterol between the two fractions, analogous to that recently described between VLDL and LDL.