SummaryThe postmortem changes (0.5, 1, 2, 3, 5, 7 and 10 hours) in pyramidal neurons of the area CAI of the hippocampus of rats have been studied with the light and electron microscope.1. (light microscopy) In the pyramidal neurons that have undergone 0.5‐ and 1‐hour postmortem autolysis (postmortem autolysis for 0.5 and 1 hours), no remarkable changes were found. In the neurons which had undergone 2‐ or 3‐hour postmortem autolysis, nuclei were slightly edematous and the perineuronal spaces were slightly disintegrated. In the neurons which had undergone more than 5‐hour postmortem autolysis, these changes were more prominent.2. (electron microscopy) The aggregation of the nuclear chromatin was already observed in the neurons which had undergone 0.5‐ or 1‐hour postmortem autolysis. In the cytoplasm, the swelling of mitochondria, Golgi vesicles and the disintegration tion of myelin sheath were partly observed in the neurons which had undergone 0.5‐hour postmortem autolysis. Within 1‐hour postmortem period, the dilatation of the endoplasmic reticulum in the neurons and the swelling of the processes of astrocytes were distinguishable. These alterations proceeded gradually in the course of time. With regard to the postmortem changes, the autolysis and the other artefacts such as pH, osmolality and concentration of the fixative and hypoxic or ischaemic fact