The actions of polycyclic aromatic hydrocarbons and glucocorticoids to regulate the synthesis of cytochrome P-450 c (the major isozyme induced by polycyclic aromatic hydrocarbons) were investigated in fetal rat hepatocytes maintained in primary monolayer culture. Treatment of hepatocytes in culture with 1,2-benzanthracene resulted in a 50-fold increase in 7-ethoxycoumarin O-deethylase activity. The level of P-450 c increased in the cells in a time-dependent fashion as determined by immunoelectrophoretic analysis. The inductive effect of BA was potentiated approximately 1.6- to 2.3-fold when 1 μ m dexamethasone was included in the culture medium. However, dexamethasone alone had little or no effect on the induction of P-450 c. The rate of synthesis of P-450 c was examined by immunoisolation of the specific isozyme from total cellular proteins radiolabeled with [ 35S]methionine and from the protein products formed during in vitro translation of the isolated mRNA. In addition, the amount of mRNA specific for cytochrome P-450 c was determined by Northern blot analysis of RNA extracted from cultured cells. The changes in the rates of synthesis and mRNA levels were found to parallel the changes in enzyme activity. The concentration of dexamethasone required to cause a half-maximal increase in P-450 c content in the presence of 1,2-benzanthracene was between 10 −8 and 10 −7 m. It is concluded that glucocorticoids act synergistically with polycyclic aromatic hydrocarbons to increase the levels of P-450 c expressed in the fetal rat liver, and that this action is likely mediated by the classical type II glucocorticoid receptor.