The renal structure in rats subjected to a 14 per cent, third degree sublethal thermal injury was studied by light and electron microscopy. Haemolysis with resultant haemoglobinuria was found to be associated with the presence of haemoglobin-like material in Bowman's spaces, tubular lumina and in the tubular cells. The findings indicated that haemoglobin was absorbed by endocytosis and was transported to, and digested within, the lysosomes of the tubule cells in the same way as after intravenous injection of homologous haemoglobin. There were no significant morphological signs of tubular degeneration or necrosis, and no intravascular fibrin thrombi were encountered at any of the post-burn intervals studied. The findings are discussed with particular reference to the changes in renal function following a burn.