Lipid synthesis from acetoacetate and 3-hydroxybutyrate in chick kidney and duodenal mucosa showed a clear decrease between 15 and 19 days of the embryonic phase, followed by an increase at hatching and a new decrease during the first neonatal period. The hepatic synthesis of lipids presented a different profile: a peak at 19-day-old embryo and a new increase during postnatal development. 2. These changes would be related to those in 3-hydroxybutyrate concentration in chick plasma throughout the perinatal period. 3. Phospholipids were the main kind of lipid formed in the three tissues. An appreciable percentage of radioactivity was also recovered as free cholesterol, especially during the embryonic phase. Triglycerides were also formed from acetoacetate in a high proportion in liver from neonatal animals. 4. Chick kidney showed the maximal ability to incorporate both precursors into amino acids. The peak obtained around hatching time would be related to the availability of the substrates. 5. Ketone body oxidation to CO2 was also maximal in kidney. In this tissue, a drastic decrease was observed during the final embryonic period, followed by a strong increase at day 1 after hatching and a new decrease at 4 days.
