The culture of fetal hepatocytes at high cell density for 64 h in medium supplemented with 5 mM glucose produced an induction of glucose-6-phosphate dehydrogenase (G6PD) mRNA in a time-dependent manner. Insulin and triiodothyronine (T3), separately, increase G6PD mRNA expression, producing an additive effect at 64 h when combined. Glucagon and, to a greater extent, dibutyryl-cAMP decreased the G6PD mRNA expression observed in the presence of 5 mM glucose and T3. Dexamethasone repressed the G6PD mRNA expression induced by glucose and insulin and decreased this expression when induced by T3, regardless of the presence of insulin. At low cell density, EGF in the presence of dexamethasone induced in parallel DNA synthesis, G6PD mRNA content, and specific activity, while EGF failed to increase these parameters at high cell density. In addition, G6PD expression in proliferative fetal hepatocytes was unresponsive to lipogenic hormones.