An excellent correlation was found between salivary testosterone (T) and serum T concentrations, as measured by RIA. Using polyacrylamide gel electrophoresis, we have demonstrated that sex steroid-binding globulin could not be identified in the saliva of men with serum sex steroid-binding globulin. After exogenous T administration, saliva and serum T rose abruptly and in parallel. Salivary T concentrations in male patients with thyrotoxicosis were similar to those in normal males, whereas the serum T and sex steroid-binding globulin values were significantly higher in the hyperthyroid patients. This study demonstrates that salivary T levels may be used as an index of free serum T.