The activity of phosphoenolpyruvate carboxykinase (GTP) in Reuber H-35 cells was decreased after the removal of 6-N,2-O-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cyclic AMP) from the medium. The decrease in activity was shown immunochemically to be the result of a rapid cessation in specific enzyme synthesis, occurring with a half-time of 40 min. The removal of dexamethasone, a less potent inducer of the enzyme in these cells, did not effect the activity of P-enolpyruvate carboxykinase or its rate of synthesis. Insulin added to either dibutyryl cyclic AMP or dexamethasone-treated cells produced a decline in specific enzyme synthesis which was not as rapid as that observed upon removal of dibutyryl cyclic AMP. This effect of insulin did not require the presence of glucose in the culture medium. Estimates of the half-life of the mRNA for P-enolpyruvate carboxykinase using actinomycin D and cordycepin suggested that after the inhibition of transcription of mRNA, enzyme synthesis continued for periods considerably longer than that observed after deinduction caused by removal of dibutyryl cyclic AMP. In addition, the synthesis of the enzyme could be restimulated by dibutyryl cyclic AMP in the absence of RNA synthesis. It was proposed that the deinduction of phosphoenolpyruvate carboxykinase in these cells is being regulated at the post-transcriptional or translational level.