||A sequential study was performed to investigate the occurrence of cell death in preneoplastic and neoplastic liver cells of F-344 rats. The animals were administered a single initiator dose of 1,2-dimethylhydrazine and were then subjected to a liver carcinogenesis promotion regimen, consisting of a diet containing 1% orotic acid. Cell death, morphologically similar to that described as apoptosis, was evident in foci of preneoplastic hepatocytes at 10 weeks after orotic acid feeding. An increased frequency of apoptotic bodies was observed in nodules, but not in the surrounding liver, 20 weeks after starting the dietary regimen, and in hepatocellular carcinomas that developed after 1 year of continuous promotion. Occurrence of this type of cell death was also observed in liver foci of rats subjected to two other promoting regimens, suggesting, thus, a possible relevance of apoptosis to the carcinogenic process in the liver.