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An exchange of phospholipids and certain peptides among various classes of rat serum lipoproteins has been demonstrated and its nature has been investigated. [32P]-Phospholipid from isolated VLDL, prepared in vivo, was transferred to HDL, and to a much lesser extent to LDL, in vivo and in vitro. This difference between HDL and LDL can be abolished by ultracentrifugation of the serum at d 1.21. Unlabeled VLDL acquired [32P]phospholipid from HDL of serum. Phospholipid associated with the α-lipoprotein component of VLDL exchanged more readily than that associated with the β-lipoprotein component of VLDL. Generally, the phospholipid species exchange in proportion to their distribution in the lipoproteins.Radioactivity from 3H-labeled protein of VLDL was transferred to HDL while HDL 3H-labeled protein in serum was transferred to VLDL during a 20-min incubation. LDL was not involved in the transfer of protein. Protein associated with the α-lipoprotein component of VLDL exchanged more readily than that associated with β-lipoprotein component. Analysis of tritiated apoproteins of VLDL and HDL by polyacrylamide gel electrophoresis revealed that three of the six peptide bands of apo-VLDL exchanged between VLDL and HDL.The data raise the possibility that intact subunits of the VLDL and HDL are being exchanged.