Cytochrome P-450 in hyperplastic liver nodules was measured during hepatocarcinogenesis with N-2-fluorenylacetamide in the rat. Cytochrome P-450 in liver microsomes decreased in the early stages of hepatocarcinogenesis. The concentration of cytochrome P-450 in the hyperplastic nodules and their surrounding tissues was examined, using homogenate from each of them. Its amount was significantly less in the hyperplastic nodules at 13 wk than in surrounding tissues, and became greater in hyperplastic nodules at 19 wk. Hyperplastic nodules of animals treated with phenobarbital showed almost the same amount of cytochrome P-450 as controls. Cytochrome P-450 in hepatoma tissues also showed similar values to hyperplastic nodules.