1 Introduction.- 2 Materials and Methods.- 2.1 Material.- 2.2 Qualitative Histochemistry.- 2.2.1 Light Microscopy.- 2.2.2 Ultracytochemistry of Aminopeptidase A.- 2.3 Quantitative Enzyme Histochemistry of Aminopeptidase A.- 2.3.1 Determination of the Absorption Spectrum of the Azo Dye from 4-Methoxy-2-naphthylamine and Purest-Grade Fast Blue B.- 2.3.2 Enzyme-Kinetic Studies.- 2.3.3 APA Activity Pattern Along the Rat Nephron.- 2.4 Biochemistry of APA and APM.- 2.4.1 Fluorometry of Renal Homogenate from Male Rats.- 2.4.2 Photometry of Renal Homogenate from Male Rats.- 2.4.3 Fluorometric Measurements of Microdissected Glomeruli (Rat).- 2.5 Methods of Investigating Kidneys from Experimental Animals.- 2.6 Chemicals and Suppliers.- 2.7 Abbreviations.- 3 Results.- 3.1 Methodological Studies.- 3.1.1 Qualitative Light-Microscopic Histochemistry.- 3.1.2 Ultracytochemistry.- 3.1.3 Quantitative Histochemistry.- 3.1.4 Biochemistry of APA and APM.- 3.2 Histochemical Localization of APA and APM in the Normal Kidney.- 3.2.1 Juxtaglomerular Apparatus.- 3.2.2 Renal Corpuscle.- 3.2.3 Proximal Tubule.- 3.2.4 Additional Reaction Sites of APA and APM.- 3.3 Animal Experiments.- 3.3.1 Low-Sodium Diet.- 3.3.2 High-Sodium Diet.- 3.3.3 Adrenalectomy.- 4 Discussion.- 4.1 Methodological Aspects.- 4.2 Animal Experiments.- 4.2.1 Juxtaglomerular Apparatus.- 4.2.2 Renal Corpuscle.- 4.2.3 Proximal Tubule.- 4.2.4 Renal Homogenate.- 4.3 Conclusion.- 5 Summary.- References.