Intraoperative radiation therapy (IORT) was delivered to remnant rat liver after partial hepatectomy to determine the chronic effects of treatment on survival, blood chemistry, liver weight, and histology. Survival at one year was 100%. Remnant liver weight was markedly increased in all animals. Liver function appeared to be unaltered in all groups and at all observation times. Inflammatory cell infiltration occurred immediately after treatment in all animals, showing a slight progression until day 45; by day 180 the values had returned to baseline. Vascular changes were seen early in all groups, then progressively decreased; the vascular score was back to baseline at days 180 and 365. Nuclear alterations were observed in both irradiated and nonirradiated hepatic cells; in all cases these were limited to isolated or focal areas of hepatocytes. There was little fibrosis formation and by day 180 all scores were back to baseline. We conclude that the chronic effects of whole liver IORT after one-third hepatectomy are minimal in the rat and are similar to those observed after surgery alone.