Ultracytochemical studies of the Aminopeptidase A (APA; angiotensinase A; E.C. 3.4.11.7) were performed on perfusion fixed rat and mouse kidneys (low concentration aldehydes) using simultaneous azo coupling with alpha-L-Glu-MNA as substrate and HPR as well as HNF as coupling agents. The studies of glomeruli and juxtaglomerular apparatus (JGA) show that APA is localized mainly at the cell membranes of podocytes and endothelial cells (rat and mouse) and of epitheloid cells (mouse) and Goormaghtigh's cells (rat). Increased APA activities are found in the region of cell contacts of epitheloid cells (mouse) and Goormaghtigh's cells (rat). In the epitheloid cells of mice, reaction product is also observed intracellularly in lysosomal structures. Concerning the functional significance of APA in the glomerulus and JGA, it would appear that this enzyme modifies or regulates angiotensin effects in the glomerulus and JGA through the angiotensin degradation.