Aldosterone-producing adenoma (APA) and idiopathic hyperplasia (IHA) are two main causes of primary hyperaldosteronism, which differ in the modes of treatment. Some of the prohormones, such as 18-hydroxycorticosterone, are elevated in adenomas. 19-Nor-deoxycorticosterone (19-nor-DOC), produced in the kidney, has been shown to be excreted in excess in patients with APA. Deoxycorticosterone is a known precursor of aldosterone and 19-nor-DOC. This study is designed to evaluate the levels of prohormones in adrenal venous effluent in eight patients, three with APA, of which two were confirmed by surgical pathology and four with IHA, and one patient with primary adrenal hyperplasia. 19-OH-DOC, a precursor of 19-nor-DOC, was found to be the main prohormone in adrenal venous effluent in patients with both APA and IHA. 19-Oic-deoxycorticosterone and 19-nor-DOC were also detected, but in smaller quantities. 19-OH-DOC appears to be the main prohormone in adrenal venous effluent for the biosynthesis of 19-nor-DOC.