Previous studies have shown that isolation and primary culture of rat hepatocytes in a standard, chemically defined medium is associated with selective changes in microsomal function. These changes were found to be selectively sensitive to addition of hormones to the culture medium. The concentration of cytochrome P-450 declined dramatically during the first 24 hours of incubation. However, cytochrome P(1)-450, a form of the hemoprotein induced by polycyclic aromatic hydrocarbons, was resistant to this change. Cytochrome P(1)-450 levels selectively rose during the first ten hours in culture and, thereafter, declined at a less rapid rate than did the cytochrome P-450 in normal hepatocytes or in cells prepared from phenobarbital pretreated animals. Addition of dexamethasone to the medium at the time of cell plating partially prevented the fall of cytochrome P-450 and of (14)C-heme in microsomes prepared from hepatocytes derived from rats given 5(14)[C]-δ-aminolevulinic acid. This suggests that the steroid decreases degradation of the hemoprotein. As compared to the loss of cytochrome P-450 in cultures of normal hepatocytes, the hemoprotein fell to lower levels in hepatocytes prepared from regenerated liver four days after partial hepatectomy. This result may be related to the accelerated formation of the monolayer in the cultures of regenerated hepatocytes. Both sn-glycerol-3-phosphate acyltransferase activity and glycerol kinase activity declined in the first 24 hours of culture. The fall in the latter enzyme was partially prevented by addition of estradiol. Collagen prolyl hydroxylase, a newly discovered microsomal constituent of the hepatocyte, rose slightly during the first 24 hours in culture. This change was augmented threefold by addition of insulin to the medium. We conclude that the present hepatocyte culture system with its attendant changes in functional phenotype may be useful in better defining the role of hormones in modulating metabolic processes in the liver.