We have used immunohistochemical, immunoblotting and messenger RNA blotting approaches to study the distribution and quantitation of three cytochrome P-450 enzymes, namely P-450 IA2, P-450 IIC and P-450 IIIA and, for comparison, epoxide hydrolase and NADPH-cytochrome P-450 reductase in human liver. Age-related changes in both the amounts and the intralobular distributions of these enzymes were demonstrated, and the enzymes differ in these regards: In fetal liver, P-450 IA2 and P-450 IIC were very low, when present at all, whereas P-450 IIIA, epoxide hydrolase and the reductase were already abundant and found in all the hepatocytes. During the postnatal period, P-450 IIC dramatically increased and was observed in all hepatocytes, the centrilobular ones being more intensely stained. P-450 IIIA was restricted to centrilobular and midzonal hepatocytes in normal adult liver. P-450 IA2 showed this same intralobular distribution; however, its presence was detected only several weeks or months after birth as judged both by immunohistochemical and immunoblotting techniques. Epoxide hydrolase and NADPH-cytochrome P-450 reductase were easily visualized in all hepatocytes regardless of the age of the donor; in child and adult livers, centrilobular hepatocytes were more intensely stained. Immunoreactive protein contents and corresponding messenger RNA levels correlated well with immunohistochemical observations. No major modification was seen in fibrotic liver, whereas both positive and negative cells were observed in cirrhotic liver nodules for all enzymes studied.