PIP: The biological activity of circulating sex steroids is dependent upon the relative binding to (SHBG) sex hormone-binding globulin. Only that fraction which is not specifically bound to the high affinity binding protein is available to the receptors or for metabolism. (D-Ng) Levonorgestrel, a synthetic gestagen used for contraception decreases the hepatic production of SHBG. Volunteers wearing contraceptive vaginal rings containing (E2) estradiol and d-Ng were studied to determine the changes in sex steroid binding which occur when d-Ng is administered in a sustained release fashion. The % of steroids not specifically bound to SHBG was measured by fractionating the serum proteins with ammonium sulfate precipitation. A binding capacity assay was used to quantitate SHBG. During the normal menstrual cycle in control subjects, the unbound fractions of both E2 and (T) testosterone remained constant at about 25% of total E2 and 10% of total T. During treatment with d-Ng, however, the % of unbound E2 increased to about 80% of the total, and that of T increased to about 55% of the total, significantly greater than that in the control cycles (p 0.01). SHBG was constant during the normal menstrual cycle, averaging 85.9 + or - 1.92 nM but was suppressed during the administration of d-Ng to 10.0 + or - 2.6 nM. When SHBG concentration was greater than 50 nM, the % binding of both E2 and T were independent of the concentration of this binding protein. When SHBG was suppressed below 50 nM, the % binding of E2 and T was directly related to the concentration of the binding protein. The affinity of SHBG for d-Ng allows competition with E2 and T for SHBG-binding sites at concentrations of SHBG below 50 nM. The increase in physiologically free E2 and T, therefore, may be a result of both the suppression of SHBG concentration by d-Ng as well as the competition between d-Ng and endogenous sex steroids for the decreased number of available binding sites on SHBG.