The axolotl, Ambystoma mexicanum, which has no specific calcium-containing sieve layer in the dermis, provides useful material for the study of the effect of Ca²⁺ on the development of amiloride-blockable active Na⁺ transport across the skin of amphibians. We raised axolotls in thyroid hormone or aldosterone or cultured the skin with corticoid plus one of several Ca²⁺ concentrations and found that 1) although the short-circuit current (SCC) was increased by both aldosterone and 3,3',5-triiodo-L-thyronine in vivo, only corticoid was necessary for such an increase in vitro; 2) the development of the SCC in vitro was both corticoid and Ca²⁺ dependent, because the SCC was well developed with over 100 µM Ca²⁺ but not with under 10 µM Ca²⁺ in the presence of corticoid, nor even with 300 µM Ca²⁺ without corticoid; and 3) Ca²⁺, but not corticoid, was necessary for the formation of cell-to-cell junctions, because the resistance of the skin was well developed with 300 µM Ca²⁺ without corticoid.