The maximum 30-fold level of fatty acid synthase (FAS) mRNA was achieved by 6hr after intraperitoneal injection of insulin. The kinetics and maximum effect of insulin were most evident on he 7.2 kb mRNA. In six hors after insulin administration there was about 100-fold increase in stead-state mRNA level. We observed a sharp decrease in 7.2kb mRNA by 8hr after insulin administation while there was no change in FAS mRNA content between the 6hr and 8hr-sampling periods. In contrast, a maximum induction of 4-fold was shown in the level of 5.1kb mRNA after insulin injection in streptozotocin-diabetic mice.