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 corre-sponding to the observed biological activity. 2. Intact rats subject to surgical stress secrete aldosterone (aldosterone-like material) equi-valent to about 45 \g=m\gDCA, and 150 \g=m\gcorticosterone/adrenal/kg/hr, with a ratio of 0\m=.\30/1. Treatment with DCA for 7 days or feeding a potassium-deficient diet for 4 weeks reduces aldo-sterone by more than 90 % without altering corticosterone; the ratio is reduced to about 0\m=.\014/1 after induction of the potassium deficiency, and to 0\m=.\044/1after DCA treatment. 3. Hypophysectomized rats secrete about 7\m=.\8\g=m\gDCA equivalent of aldosterone and 2 \g=m\g corticosterone, giving a ratio of 3\m=.\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 onl