In a series of studies to elucidate the direct effect of irradiation on liver, rats were Co-60 irradiated to the right side of liver in a dosage of 600 R. The results obtained were as follows. 1. Activities of serum transminase were significantly elevated in the rats 2 and 4 weeks after irradiation, and normalized after 6 weeks. 2. Alkaline phosphatase activities were within normal ranges except for the significant elevation 2 weeks after irradiation. 3. The main feature of histological changes was loss of intracytoplasmic particles followed by vacuolar degeneration of hepatocytes. It appeared markedly 2 weeks after irradiation and normalized after 6 weeks. Veno-occlusive lesion observed in human model was not the main feature in the rats. 4. Depletion of the glycogen particles in hepatocytes was noticeable 3 days after irradiation. reached to the maximum 2 weeks and returned to be normal after 6 weeks. 5. Fatty droplets of hepatocytes were seen in a few rats 6 and 8 weeks aft r irradiation, which were mild and restricted to the periportal region.