Radioactively labeled triacylglycerols, 1,3-dioleoyl-2-[1-¹⁴C]-palmitoylglycerol and 1,3-[9,10-³H]-dioleoyl-2-palmitoylglycerol, were incubated with dogfish (Squalus acanthias) serum for periods of up to 10.0 hr. Changes in the positional distributions of carbon-14 and tritium within the triacylglycerols in 5.0 hr and 10.0 hr indicate that intermolecular and intramolecular shifts occur among the fatty acids. In addition, a maximum of 8.3% of the carbon-14 and 5.9% of the tritium was incorporated into 1-alkyl-2,3-diacylglycerols; essentially all of this incorporated radioactivity was associated with the acyl chains in the 1.5 and 5.0 hr incubations. In the 10.0 hr incubations, however, 25% of the tritium incorporated into the 1-alkyl-2,3-diacylglycerols was associated with the 0-alkyl chains. Radioactivity was not incorporated significantly into free fatty acids in the 1.5 and 5.0 hr incubations. These results indicate that acyl transfer reactions take place among molecular species of triacylglycerols, as well as between triacylglycerols and 1-alkyl-2,3-diacylglycerols. In the latter case, the conversions appear to be operative in the virtual absence of net biosynthesis.