Livers of insulin-treated diabetic rats accumulate albumin and malic enzyme mRNAs at very different rates. We now report that in normal rats insulin directs a specific increase in malic enzyme mRNA, while albumin mRNA levels remain unaltered. These studies support the contention that insulin regulates the accumulation of hepatic mRNAs in a highly specific manner. To evaluate whether or not albumin and malic enzyme mRNA levels are determined by altered rates of transcription, in vitro transcription assays were performed. The results of these studies demonstrate that increased malic enzyme mRNA levels in insulin-treated normal rats and increased malic enzyme and albumin mRNA levels in insulin-treated diabetic rats do not involve altered rates of transcription of the genetic sequences encoding these proteins. For these two specific proteins, insulin mediates changes in mRNA levels by a post-transcriptional mechanism.