In rat kidneys the early post mortem changes of the cortical tubules and of the peritubular capillaries were investigated after different forms of preparation. In the instantly frozen, freeze-dried tissue, a systematic, correlated orientation of the tubules in the renal cortex was found. The main axis of the tubules was orientated radially out to the surface of the kidney, and the corresponding peritubular capillaries were orientated parallel to the tubules. The total interstitial volume in the kidney cortex was found quantitatively significant (21 per cent), including the capillary volume (7 per cent). This was compared to the conventional fixed tissue, in which the total interstitial volume was 9 per cent, and in which the capillaries were not readily recognized. In instantly frozen, freeze-substituted tissue slight artificial changes were found, but the parallel orientation of the tubules and the capillaries could still be recognized. In freeze-substituted tissue, frozen 2 minutes after the removal of the kidney and in conventionally fixed tissue, the parallel, radial orientation of the tubules and capillaries was distorted.