The metabolism of triglyceride in the plasma low density lipoproteins (LDL) and high density lipoproteins (HDL) was studied in the normal-fed rabbit. Preparations of LDL and HDL labeled with 3H in the triglyceride moiety were obtained from donor rabbits that had previously been injected with 3H-palmitic acid. These labeled lipoproteins were subsequently either reinjected into other rabbits or incubated in vitro with unlabeled lipoprotein fractions. In vivo, there were bidirectional transfers of 3H-triglyceride between the LDL and HDL, the rate of which considerably exceeded that of any irreversible removal from the plasma compartment. To a much lesser extent, there was also a transfer of 3H-triglyceride from both LDL and HDL to very low density lipoproteins (VLDL). In vitro, so long as the dialysed 1.21 g/ml infranate of serum was present there was a significant transfer of 3H-triglyceride from both LDL and HDL into each of the other lipoprotein fractions that in no case could be accounted for by a corresponding net transfer of triglyceride mass. It was concluded that the pools of triglyceride in LDL and HDL, and to a lesser extent in VLDL, comprise parts of a larger, progressively equilibrating pool.