Cryostat sections from the mesonephros of various pig embryos with a crown-rump-length of between 17 and 95 mm were used for light microscopical assays of acid hydrolases (acid phosphatase, beta-D-N-acetylglucosaminidase, beta-D-glucuronidase), oxidoreductases (succinic dehydrogenase, NADH- and NADPH-tetrazolium reductase) and adenosine triphosphatases (Mg2+- and Na+--K+-ATPase). Our main intention was to distinguish more accurately between the different parts of the pig's nephron, which is exceedingly long and coiled. The proximal tubule, that exhibits a high activity for acid phosphatase but none in beta-D-glucuronidase incubations, shows no subsegmentation apart from a stronger reaction of its initial segment that was apparent in three of our assays. In the distal tubule, a preattachment convolution, an attachment zone, and a postattachment coil can be discriminated by a synopsis of all histiograms. The beginning of the collecting tubule is situated in the middle of the organ and not at its dorsal face as was previously believed. Up to three different segments can be discriminated in the collecting tubule. The distal and the collecting tubule harbor on ouabain-sensitive Na+--K+-ATPase activity which decreases considerably towards the Wolffian duct. The enzymatic maturation of the mesonephric pig nephron is almost completed in 17 mm embryos.