The objective of this project is to characterize several factors that may be important in the initiation phase of hepatocarcinogenesis in adult rats: 1) the phase of the cell reproductive cycle at which carcinogen-induced damage is incurred; 2) DNA repair; and 3) elongation of the cell-cycle attributable to DNA damage. Cell-cycle-dependent variation in hepatocyte susceptibility to initiation of carcinogenesis will be studied by treating partially hepatectomized rats with a single, sublethal dose of one of three model carcinogens (X-radiation, methylacetoxymethylnitrosamine, and benzo(a)pyrene diol epoxide). At times when proliferating hepatocytes are in defined phases of a synchronized cell cycle. Hepatocellular tumors will be enumerated after an appropriate interval of feeding of the liver tumor promoter, phenobarbital. Cell-cycle extension in damaged livers will be evaluated by charting the fractions of hepatocytes in the S and M cell cycle phases at various times after the carcinogen treatment. Production of damage in hepatic cell DNA and repair of that damage will be characterized using radiolabelled chemical carcinogens for chromatographic analysis of DNA lesions, by radioimmunoassay of unlabelled carcinogen-DNA adducts, or by prelabelling liver DNA when rats are neonates and then quantifying the X-ray-induced DNA strand breaks by alkaline strand separation techniques. These studies should further clarify how proliferating hepatocytes vary in susceptibility to initiation of carcinogenesis according to their location in the cell cycle when damaged by carcinogen. DNA repair and cell-cycle-extension studies will be used to determine whether the variation in sensitivity is correlated with the net damage persisting in DNA when it is replicated.