Both pregnenolone and 17-OH-pregnenolone were found to be higher in the plasma of patients with poorly controlled congential adrenal hyperplasia than in normal subjects. The plasma levels of these precursor steroids were significantly correlated with urinary 17-ketosteroid and pregnanetriol excretion and with plasma testosterone. The mechanism where by plasma pregnenolone and 17-OH-pregnenolone levels are elevated in patients with 21-hydroxylase deficiency is unknown, but the phenomenon of product inhibition is suggested as a possible explanation. As 17-OH-pregnenolone in plasma is almost entirely of adrenal origin, its measurement promises to be useful in the management of patients with congenital adrenal hyperplasia. Acute stimulation with ACTH caused negligible changes in the plasma levels of pregnenolone and 17-OH-pregnenolone and failed to distinguish between overly, appropriately, and under-treated patients. However, following repeated stimulation with repository ACTH, the steroid levels rose. These findings indicate limited adrenal responsiveness to ACTH following chronic glucocorticoid treatment of congenital adrenal hyperplasia, even in under-treated patients, and suggest that normal precursor steroid levels in plasma and normal 17-ketosteroid and pregnanetriol excretion can only be achieved by the suppression of total steroidogenesis to less than that occurring in normal subjects.