The localization of microsomal alanine aminopeptidase was investigated in the rat kidney. Resin embedding failed to demonstrate the localization of the enzyme by immunogold labelling. Using a cryo-ultramicrotomy method the enzyme could be detected on the luminal side of the brush border membrane of proximal tubular cells and to a lesser degree in their mitochondria. Furthermore, vesicular structures labelled with gold were found in the cytoplasma in the apical region of these cells.