The morphological changes produced in the mesonephric tubule during ontogenesis, not previously reported in amphibians, are described in Rana ridibunda tadpoles using light and electron microscopic methods. The rudimentary nephron units do not develop synchronously along the subperitoneal nephrogenic ridged cord. The first signs of morphogenesis are the presence of round euchromatinic nuclei and mitotic figures. The subsequent developmental stages are characterized by detachment of the rudimentary nephrons from the nephrogenic cord. Renal corpuscle formation is characterized by glomerular expansion, differentiation of large fenestrated capillaries and the presence of a discrete mesangium and a small capsular space. Interstitial capillaries next to the renal corpuscle rudiments appear to induce invagination and differentiation of the capsular epithelium. Developing podocytes were cuboidal undifferentiated epithelial cells with scarce primary processes and with an extensive part of the cell surface lying flat on the glomerular basement membrane. These features reflect low or no glomerular filtration during nephron development. The ciliated neck segment and peritoneal funnels show similar structural features. The latter were not physically connected with the nephrons, but opened into renal blood vessels. Involutive peritoneal funnels were observed.