3',5'-Cyclic AMP inhibits fatty acid synthesis in mitochondria, microsomes and supernatant but does not provoke an enrichment in 3H with regard to 14C as it occurs during fasting after glucose 6-(3)H or 6-(14)C administration. Insulin does not exert any action on "controls" but restores lipogenesis in fasted animals in the examinated fractions and normalises the 3H/(14)C ratio, even that of microsomes. Control of neoglucogenesis by this hormone is more likely than by 3',5'-cyclic AMP.