On addition to the solution on the side of the outer surface of the frog skin of ethacrynic acid (0·063-1·0 mg/ml) or mersalyl (0·2-1·0 mg/ml) as in the case of furosemide, the potential difference rises for 1-2 min by tens of millivolts with unchanged or increased short circuit current. This is apparently due to the block of the chloride channels of the apical membrane of the cells of the epithelium by ethacrynic acid, mersalyl and furosemide. On addition to the solution on the side of the basolateral membranes of mersalyl and ethacrynic acid, as in the case of oubaine, after a latent period the duration of which is inversely proportional to the concentration of inhibitor, the potential difference and the short circuit current diminish (half·period of drop in current 10 min and longer). Ethacrynic acid and mersalyl apparently inhibit the sodium pump of the basolateral membranes of the cells of the epithelium of the frog-skin.