Na specific passive transport channels in the apical membrane of the str. granulosum control the input step of Na transport through frog skin epithelium. Their permeability decreases with acidification of the outer bathing medium (apparent pK = 4.1, complete closure of channels at pH 3). Phosphate or COO groups of the membrane can be responsible for this behaviour. To characterize the group the carbodiimide glycine methylester method was used (Hoare and Koshland, 1967). The inward facing membranes of abdominal skins of r. temporaria and r. esculenta were depolarized with high K concentrations, such that the apical membrane determined the electrical properties of the skin. After some control recordings the outer surface was exposed for 10 min to a Na free solution containing 25 mM Carbodiimide, 75 mM glycine methylester 1 mM CaCl₂ and 5 mM TRIS, pH 3.5, then washed with Ringer's. Membrane resistance (increased) and short circuit current (decreased to zero) were now independent of pH (between 3 and 7) and outer Na concentration, just as during complete blockage of Na current by amiloride. The carbodiimide glycine methylester blockage is irreversible and acid stable (which excludes phosphate groups). Apparently the Na channels (or their immediate vicinity) contain COO groups. The channels are blocked when these groups are protonated or modified with nucleophilic reagents.