Prolactin (PRL) induced a transient increase in active Na transport measured as the short circuit current (SCC) of the tadpole skin. Increase in SCC by PRL accelerated as the stage advanced between stages XXII and XXV. Prolactin caused two types of effect on the electric parameters of the active Na transport. In type I, it increased both the electromotive force of the active Na current (ENa) and the resistance to the active Na current (RNa). In type II, it decreased both ENa and RNa. A second application of PRL had no effect on SCC; that is, desensitization to PRL was observed.