Blot analysis of poly(A+) RNA showed a rapid and dramatic insulin induction of fatty acid synthase (FAS) mRNA in diabetic mouse liver. Insulin administration to diabetic mice increased the mRNA level 4-fold within 1 h, and a maximum of 19-fold was reached in 6 h. The nutritional induction of FAS mRNA by fasting/refeeding was abolished by streptozotocin-induced diabetes and also by dibutyryl cAMP administered during refeeding in normal animals, demonstrating the roles of insulin and cAMP during nutritional manipulation. Run-on transcription analysis with isolated nuclei showed that the transcription rate of the FAS gene increased 3.5-fold after 30 min, reached a maximum of 7-fold after 2 h of insulin administration in diabetic animals, and was maintained at the maximum level to 6 h. The cAMP, administered to previously fasted normal mice during refeeding of a high carbohydrate diet, abolished the increased transcription of the FAS gene caused by nutritional manipulation. These results indicate positive control by insulin and negative control by dibutyryl cAMP of FAS gene transcription. Furthermore, the effect of insulin on the FAS mRNA level was abolished by cycloheximide administration. This indicates that ongoing protein synthesis is necessary for the transcriptional activation of the FAS gene by insulin.