Post-mortem changes in the proximal and distal tubules of rat kidneys left in situ have been studied at the light microscope and ultrastructural levels. The distal tubular nuclei showed distinct changes which were completely different from the usual nuclear appearances after death and resemble the changes seen in apoptosis. These observations suggest that the unusual changes in the autolytic distal tubular nuclei may be due to the activation of endogenous endonucleases.