Biosynthesis of 18-hydroxycorticosterone and aldosterone from radioactive progesterone, 11-deoxycorticosterone, 11-dehydrocorticosterone and corticosterone by adrenals of bullfrogs and that of normals as well as gonadectomized Rhesus monkeys was studied in vitro. The adrenals of monkeys were much less active than those of the frogs. Corticosterone and progesterone were more efficiently converted to 18-hydroxycorticosterone and aldosterone by the frog adrenals while the formation of these products from 11-dehydrocorticosterone was very little; the adrenals from the monkeys utilized the latter compound comparaticely better than the other substrates. The data support that the main pathway for aldosterone biosynthesis is progesterone:→ 11-deoxycorticosterone → corticosterone→ 18-OH-corticosterone → aldosterone. The pathway involving 11-dehydroaldosterone, proposed by some workers, is comparatively less efficient for aldosterone biosynthesis but could be involved as an alternate route and a mechanism for limiting the biosynthesis of aldosterone in mammals.