Insulin regulates the synthesis of several proteins in a variety of tissues¹. Before techniques were available to quantify the amount of specific mRNAs, insulin was thought to regulate the synthesis of proteins by influencing the rate of translation of a fixed amount of mRNA. A very different interpretation is called for by experiments which show that insulin alters the amount of several specific mRNAs^2–5, but little is known about the mechanism. Insulin decreases the rate of synthesis of the critical gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in both liver⁶ and H4IIE heptoma cells^7,8. We recently showed that insulin acts directly on H4IIE cells to decrease mRNA^PEPCK activity without any other hormone intermediaries⁸. This effect is mediated by the insulin receptor and occurs at insulin concentrations which are well within the physiological range (10⁻¹²–10⁻⁹ M)⁸. Here we extend these studies to show that insulin specifically inhibits transcription of the PEPCK gene. This inhibition results in a rapid decrease in the concentration of nuclear PEPCK transcripts which is followed, in turn, by a proportionate decline in cytoplasmic mRNA^PEPCK and synthesis of the protein.