Bilateral paratesticular tumors were observed in a 32-yr-old man 14 yr after he developed a pituitary tumor after adrenalectomy for Cushing's disease (Nelson's syndrome). Plasma ACTH concentrations were markedly elevated (mean, 6350 pg/ml), but urinary free cortisol concentrations were low (27-31 micrograms/24 h). Catheterization revealed a spermatic to peripheral venous gradient for cortisol consistent with secretion of this steroid by the tumor. This was confirmed by decreased cortisol excretion after tumor excision. Serum androgen (testosterone, androstenedione, dihydrotestosterone, and dehydroepiandrosterone-sulfate) and progestin (progesterone and 17-hydroxyprogesterone) concentrations were decreased and did not decline further after tumor removal. These latter observations suggested that the paratesticular tumors did not secrete appreciable testosterone or any of its immediate precursors. Serum gonadotropin levels were also low. Cyproheptadine treatment resulted in a marked lowering of plasma ACTH concentrations (221-320 pg/ml). This was associated with an increase in both plasma LH and testosterone concentrations. These observations are consistent with the hypothesis that ACTH may directly affect LH and testosterone secretion.