The effect of aldosterone on the colonic transmucosal potential difference (p.d.) was examined in normal, adrenalectomized and Na+ depleted rats. Continuous intravenous infusion of aldosterone in conscious animals was more effective than single i.v. injection and over the dose range 0·5–50 μg./hr. for 6 hr., p.d. increased linearly with log dose and returned to the pre-infusion levels 16–24 hr. after stopping infusion. Only a region 2–4 cm. from the anus responded to low doses, but with high doses an increasing length of colon responded. Cortisone (250 μg. daily) intramuscularly maintained adrenalectomized rats even when severely Na+ depleted without affecting the p.d. Cortisone (2·5 mg. daily, i.m.) raised the p.d. but the effect of low doses of aldosterone was unchanged. Neither Na+ depletion nor pentobarbitone anaesthesia affected the action of aldosterone but large doses of spironolactone reduced it. Aldosterone was effective when applied locally by submucosal injection in very low dose but was almost ineffective even in very large dose in the lumen. Again the most sensitive region was 2–4 cm. from the anus. Ouabain (10-3m) injected into the submucosa rapidly reduced the increased p.d. but was ineffective in the lumen.