SUMMARY: 1. A potent mineralocorticoid has been detected in a paper-chromatographic fraction of extracts of blood from the rat's adrenal vein. The active fraction contains a substance with the same RF and soda-fluorescence reaction as aldosterone, and is present in a concentration corresponding to the observed biological activity. 2. Intact rats subject to surgical stress secrete aldosterone (aldosterone-like material) equivalent to about 45 μg DCA, and 150 μg corticosterone/adrenal/kg/hr, with a ratio of 0·30/1. Treatment with DCA for 7 days or feeding a potassium-deficient diet for 4 weeks reduces aldosterone by more than 90% without altering corticosterone; the ratio is reduced to about 0·014/1 after induction of the potassium deficiency, and to 0·044/1 after DCA treatment. 3. Hypophysectomized rats secrete about 7·8 μg DCA equivalent of aldosterone and 2 μg corticosterone, giving a ratio of 3·65/1. In rats 2 days after hypophysectomy, or in rats 7 or 14 days after hypophysectomy and injected daily with corticotrophin, acute stimulation with corticotrophin leads to a greater proportional increase in the secretion of corticosterone than that of aldosterone and results in levels of both comparable with those in intact rats. In the rats 7 days after operation but not maintained on corticotrophin, acute corticotrophin stimulation (see text, p. 134) raises aldosterone to the levels in intact rats, but corticosterone is increased only in about the same proportion as aldosterone and the ratio remains similar to that in untreated hypophysectomized rats. 4. It is concluded that the rates of secretion of aldosterone and corticosterone by the rat adrenal vary independently, the ratio depending on the state of the animal. 5. In a single experiment, treatment of hypophysectomized rats with DCA and corticotrophin for 7 days resulted in inhibition of aldosterone secretion analogous to the effect observed in intact rats. This experiment suggests that the action of DCA can occur in the absence of the pituitary. It is possible also that the effect of potassium deficiency on adrenal secretion can occur in the absence of the pituitary.