Aldosterone biosynthesis by quartered adrenal glands from rats with different forms of experimental renal hypertension due to clamping of one renal artery, was studied under various in vitro conditions. During incubation without aldosterone-stimulating substances, the adrenals of rats with one renal artery clamped and the other kidney left intact produced 200 % more aldosterone from endogenous precursors and converted 50 % more added tritium-labelled pregnenolone, progesterone or corticosterone to aldosterone than adrenals of control animals. The difference in aldosterone production was less marked when serotonin, KCl or ACTH was added to the incubation medium. The production of corticosterone and of deoxycorticosterone, respectively, was almost the same in adrenals of both groups of rats under most in vitro conditions. The marked rise in aldosterone production seen in the presence of an intact contralateral kidney was partially or completely inhibited, when simultaneously with renal artery constriction, the contralateral kidney was removed or the ureter of either the clamped or the contralateral kidney was ligated. These results indicate that in rats with experimental renal hypertension, increases in aldosterone production are correlated with increases of plasma renin activity and of the renin content of the clamped kidney, but are independent of changes in blood pressure. Since chronically elevated levels of plasma or renal renin activity act mainly in the early stages of aldosterone biosynthesis, it is concluded that the marked activation in the final stages of aldosterone biosynthesis observed in sodium deficiency is not mediated by the renin-angiotensin system.