Standardized bilateral ovarian-adrenal vein catheterization was utilized to preoperatively assess glandular steroid release in seven consecutive cases of occult virilizing gonadal neoplasms. Peripheral testosterone (T) exceeded 1.5 ng/ml in all instances (range, 1.51 to 8.67 ng/ml). Endoscopy and radiography failed to locate the functional lesions. Catheterization showed a unilateral elevation of the ovarian-peripheral vein gradient for T greater than 2.7 ng/ml in six women. In the remaining patient, gradient analysis ruled out an adrenal tumor but did not facilitate lateralization of the gonadal lesion due to subselective ovarian effluent sampling. In addition to the consistent hypersecretion of T, variable excess gonadal output of dihydrotestosterone, androstenedione, dehydroepiandrosterone, and 17 alpha-hydroxyprogesterone was evident. Associated adrenal androgenic hyperfunction was documented in three subjects. Histologic evaluation of the implicated ovaries revealed three lipid cell, two Leydig cell, and two Sertoli-Leydig cell tumors, respectively, measuring between 0.6 and 2.2 cm in diameter. No correlation was found between any of the following parameters: peripheral or glandular vein steroid levels, androgen gradients, severity of symptoms, tumor morphology, and tumor size. In conclusion, appropriate application of selective catheterization may considerably reduce the frequency and extent of operative intervention.