Requ le 22 fevrier 1969 3',5'-Cyclic AMP inhibits fatty acid synthesis in mitochondria, microsomes and supernatant but does not provoke an enrichment in sH with regard to r4C as it occurs during fasting after glucose 6-sH or 6-r% administration. Insulin does not exert any action on "controls" but restores lipogenesis in fasted animals in the examinated fractions and normalises the aH/ r4C ratio, even that of microsomes. Control of neoglucogenesis by this hormone is more likely than by 3',5'-cyclic AMP.