A B S T R A C T A partial testicular defect in testosterone secretion has been documented in a pubertal male with a congenital adrenal hyperplasia due to hereditary deficiency of the A5-isomerase-3fi-hydroxysteroid dehydrogenase enzyme complex (AY-3j-HSD). Diagnosis of the enzymatic defect is based on the clinical picture of ambiguous genitalia and salt-losing crisis in infancy, together with high urinary A5-pregnenetriol and plasma dehydroepiandrosterone when the patient was taken off replacement corticoid treatment. No hormonal response to ACTH or salt deprivation was demonstrable. In addition, in vivo studies revealed a partial enzymatic defect in the testis. Although plasma testosterone was low-normal (250 ng/100 ml), plasma A5-androstenediol was markedly elevated and rose to a greater extent than testosterone after human chorionic gonadotropin administration. In vitro testicular incubation studies suggested a testicular A5-3%-HSD enzyme defect with less A4 products formed from A5 precursors than in a control testis. Histochemical studies of the testis were also consistent with this defect. Testicular biopsy revealed spermatoAn abstract of a portion of this work has appeared in Pediatr. Res.