The onset of active Na+ transport and activated Cl conductance (GCl) across the skin epithelium of Pelobates syriacus was investigated during natural ontogenetic development. Structural features, including band three and Peanut lectin bindings were tested in parallel and structure-function relationships were attempted. The 22 specimens studied were divided into two tadpole, three juvenile, and two adult stages, corresponding to the Taylor-Kollros standard table, in accordance with external morphology of their developmental stage. Onset of transepithelial electrical potential and drop in conductance occurred abruptly, coinciding with metamorphosis climax of tadpoles into juveniles at about stage XXI of development. Amiloride-sensitive Na+ transport occurred a little later at stage XXIII, followed by the appearance of activated Cl conductance, GCl. Parallel structural examination showed that skin MR cells occurred upon metamorphosis, as the tadpole integument transformed into the adult epithelium and could be associated with the occurrence of activated GCl. It was not related temporally with the appearance of band three protein in MR cells. Our findings support the association of GCl with MR cells, whereas band three may only be a corollary of GCl and not necessarily essential for its manifestation. J. Exp. Zool. 298A:1–9, 2003. © 2003 Wiley-Liss, Inc.