The effect of 24 h fasting on ketone body utilization by three extranervous tissues, liver, duodenum and kidney, was studied in two critical ages of neonatal chick: 4 and 9 days. In 4-day-old chick, plasma concentration of 3-hydroxybutyrate increased about 9-fold after 24 h starvation, while in 9-day-old chick this parameter increased about 23-fold in the same conditions. Hepatic lipogenesis from both precursors sharply decreased by fasting. Changes in the lipogenic activity of duodenum were less patent. However, we have found a clear increase in lipogenesis in chick kidney after 24 h starvation. CO2 production from acetoacetate was higher than that found from hydroxybutyrate. No significant differences in the acetoacetate oxidation to CO2 was observed in any tissue assayed after 24 h fasting. 14C incorporation from ketone bodies into amino acids was clearly decreased in kidney from 9-day-old chick by fasting. In liver and duodenum, acetoacetate incorporation into amino acids was higher than that from hydroxybutyrate.