The regulation of fatty acid synthase in rat liver was investigated at transcriptional and post-transcriptional levels. When rats were fasted for 3 days and refed on a high-carbohydrate diet, the amounts of FAS in liver cytosol began to increase at 12 hours and further increased until 48 hours. The amount of mRNA for FAS began to increase at 6 hours and reached to a maximum level at 12 hours, indicating that the expression of mRNA for FAS precedes the increase of FAS protein pool. After 12 hours the amounts of mRNA gradually decreased and remained at a much lowered level between 24 and 48 hours. The elevated amount of FAS mRNA reflected on the amount of FAS protein in the first 24 hours, but these two parameters were not paralleled thereafter, probably due to the changes in the translational efficiencies. The run-on transcriptional activity of FAS gene began to increase at 4 hours after refeeding a high-carbohydrate diet and further increased to reach a maximum level 25 fold of the initial level at 12 hours, followed by a 16 fold level between 24 and 48 hours. The elevation of run-on transcriptional activity of FAS gene preceded the increase of FAS mRNA in the liver cytosol by 2 hours, and a similar increasing pattern was observed until 12 hours. However, FAS mRNA concentration decreased gradually after 12 hours, while the transcriptional activity remained at a high level until 48 hours. The changes in FAS mRNA content in the cytosol of rat liver were closely related to the transcriptional activity of FAS gene in the early phase of induction, but another regulatory mechanism seems to operate in the decrease of mRNA after 12 hours.