Plasma levels of 11-hydroxycorticosteroids (11-OHCS), deoxycorticosterone (DOC), aldosterone, Δ4-androstenedione, testosterone, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were measured following the intramuscular administration of αl-24 ACTH Zn and substituted αl-18 ACTH 1 mg in 5 dexamethasone-suppressed healthy male volunteers. There was a marked rise in the plasma concentration of 11-OHCS, DOC and aldosterone following the injection of each preparation, αl-24 ACTH Zn having the more prolonged duration of action of at least 24 hr. In contrast, in 2 patients with collagenoses treated with αl-24 ACTH Zn 0.5 mg im twice weekly for 27 and 48 months respectively, there was no rise in the plasma level of aldosterone following αl-24 ACTH Zn 1 mg im, whereas plasma deoxycorticosterone reached higher levels than those observed in the normal subjects in the 1 patient in whom it was estimated. The plasma level of the adrenocortical androgen, Δ4-androstenedione, increased, whereas the plasma level of the major androgen, testosterone, decreased after each corticotropin analogue. The fall in plasma testosterone concentration was not due to a reduction in the secretion of pituitary gonadotropins, as FSH and LH levels in the plasma were unchanged during the 24 hr of study. In a male patient with Addison's disease on the same dexamethasone regime as the normal subjects, there was a fall in the level of plasma testosterone following substituted αl-18 ACTH 1 mg im. It would appear, therefore, that ACTH in pharmacological dosage causes a fall in plasma testosterone by a direct action on the interstitial cells of the testis.