Hyporeninemic hypoaldosteronism has been shown to occur in streptozotocin-induced chronic diabetic rats with normokalemia. To test the nature of the aldosterone deficiency, we investigated the responses of aldosterone production to angiotensin II (AII), ACTH, and potassium in adrenal zona glomerulosa cells from diabetic rats at 6 weeks after an injection of streptozotocin compared with those in the cells from control rats. In diabetic rats, plasma glucose was high and plasma immunoreactive insulin was low. Diabetic rats also had low levels of PRA and plasma AII, low levels of plasma aldosterone, and normal levels of plasma corticosterone and plasma potassium. The zona glomerulosa width was narrower in diabetic rats than in control rats. Basal aldosterone production, when corrected to an uniform number of cells per group, was similar in the cells from control and diabetic rats. Cells from diabetic rats showed a less sensitive and lower response of aldosterone production to AII, increases in the threshold and the ED50, and a decrease in the maximal AII-stimulated aldosterone level. ACTH, however, caused a similar effect on aldosterone production in the cells from control and diabetic rats. Cells from diabetic rats exhibited a less sensitive response of aldosterone production to potassium and a tendency to be low in the maximal potassium-stimulated aldosterone level, presumably attributable to the impairment of adrenal zona glomerulosa cells to AII. We conclude that the hypoaldosteronism observed in our diabetic rats may be secondary to the deficiency of AII.