The Hansson histochemical method for carbonic anhydrase has been modified by embedding of the tissue in a resin before sectioning and incubation. While retaining the now generally accepted validity and specificity of the original method it has several advantages. It is simple and rapid and appears to give unusually sharp localization of the stain. The section thickness may be varied to suit light microscopy and electron microscopy with resolution down to 25 A. The method has been discussed especially with regard to false localization. The intracellular distribution of carbonic anhydrase in the kidneys of frogs, rats and birds has been examined. As a basis for the histochemical investigation an ultrastructural analysis of the avian nephron has been performed, using mainly young swifts. Thin tubules are found exclusively in medullary nephrons, and their cells are of an unusual structure combining high cell thickness with strong development of the paracellular route. The avian initial collecting tubule contains a high proportion of dark cells containing mainly baso-lateral carbonic anhydrase. The distribution and morphology of dark cells in vertebrate nephrons has been surveyed. These cells are also compared with oxyntic cells and it is suggested that the cells are responsible for the terminal acidification of urine taking place in the kidney. A simplified terminology for the vertebrate nephron is suggested.