Postmortem changes in hepatic tissues of male rats kept at 3 different temperatures after death (room temperature at 23 degrees C, body temperature at 37.5 degrees C, and cool temperature at 5 degrees C) were examined in relation to the hour of death. The time of occurrence and the degree of histological postmortem changes in the liver were found to differ depending on the temperature at which the body was kept (5 degrees C < 23 degrees C < 37.5 degrees C). There was a reduced stainability of erythrocytes, atrophy of hepatocytes, sinusoidal dilatation and karyopyknosis of endothelial or Kupffer cells, after 48 h at 5 degrees C. These observations were made after 3-5 h in tissues of rats kept at 37.5 degrees C, as compared to 12 h at 23 degrees C. Atrophy of hepatocytes and sinusoidal dilatation were prominent in the subcapsular region. In epithelia of the interlobular bile ducts (IBD), swelling of nuclei and karyopyknosis with an eosinophilic cytoplasm were evident. Electron microscopy revealed that the former change was heterogeneity of nucleoplasm and peripheral localization of nuclear chromatin with coagulative cytoplasmic organelles, and the latter was atrophy of nuclei with cytoplasm, including disruption and disappearance of organelles. The time course of these findings in dead rats kept at different temperatures suggests that postmortem changes of epithelial nuclei of IBD progressed from swelling to karyorrhexis (disappearance) via karyopyknosis. The frequency of karyopyknosis with an eosinophilic cytoplasm of IBD epithelia (positive cells) at 37.5 degrees C increased rapidly up to 5 h after death, and decreased extensively after 12 h. At 37.5 degrees C, the frequency of positive cells increased constantly up to 24 h, in accordance with hour of death. At 5 degrees C, the frequency of positive cells increased slowly up to 48 h, and the majority of epithelia showed positive changes at 72 h. The frequency of cells positive for karyopyknosis and eosinophilic cytoplasm can serve as one marker for estimating the hour of death.