The resistance of the apical membranes of toad skin (Bufo viridis) was measured during its natural moulting cycle using a fast flow technique. The skin behaved in all periods of the moulting cycle as a nearly perfect sodium electrode. In the presence of amiloride (10(-4)M), the total resistance of the same skin was identical with solutions which contained either sodium or potassium. The resistance of the skin with potassium was sensitive to amiloride in the period just after moulting. The resistance of skins which were made shunted by treating them with urea on the outside area was insensitive to amiloride in solutions containing potassium; a small effect was still observed with sodium. It is suggested that the transient sensitivity to amiloride, with potassium, is the result of differentiation of the sodium specific sites at the apical membranes of the skin.