IT IS generally recognized that the steroidal hormones excreted in urine may provide unreliable qualitative and quantitative indices of endogenous hormone production. In recent years many procedures have been restored to in an effort to resolve this difficulty. One such consists in the calculation of endogenous hormone production from the percentage of the exogenous hormone recovered in the form of its metabolites in the urine as in the case of progesterone (1). A second procedure is based on the assumption that the amount of hormone required for complete replacement therapy in insufficiency states is equivalent to normal endogenous hormone production. Attempts have also been made to estimate endogenous hormone production from the hormonal content of the venous blood from a ductless gland, specifically, in the case of the adrenal (2). It is the purpose of this study to describe another approach to this problem. This is based on the neutralization of an endogenous hormone by the administration of an appropriate oppositely acting hormone. This procedure is limited to the estimation of those hormones which produce mutually antagonistic biologic and biochemical effects.