Palmitic acid-1-C13 and tripalmitin-carboxyl C14 dissolved in olive oil were administered orally to previously fasted rats. The animals were sacrificed at 1, 2 and 3 hours after the administration of the doubly labeled lipid and the lipids were recovered separately from the stomachs and small intestines. The presence of each isotope in the lipid-fed and in that recovered was measured. The intestinal lipids showed a marked decrease in the excess of C13. The C14 label was considerably reduced in specific activity, but not to a comparable extent. The data appear best explained by a combined process of selective absorption of the free fatty acids and dilution with endogenous lipids. When C13- and C14-labeled palmitic acids were administered to an animal to determine whether any isotope effect might influence the results, a decrease in isotope content was observed; however, no change occurred in the ratio of the two isotopes.