SHBG titer (maximum binding capacity of sex hormone binding globulin for 17 beta-estradiol), percentage of plasma unbound estradiol, plasma unbound estradiol concentration, plasma total estradiol concentration and total testosterone concentrations were determined in ten normal premenopausal women to study the regulation mechanism of plasma unbound estradiol concentration and the physiological role of SHBG in the mechanism. The percent of unbound estradion was 1.48+/-0.37 (mean+/-standard deviation). The unbound estradiol concentration was 1.24+/-0.74 pg/ml. The total estradiol and testosterone concentrations were 84+/-50 pg/ml and 0.8+/-0.09 ng/ml. SHBG titer was 42+/-15 pmol/ml. SHBG-testosterone ratio was 51+/-11.9. There was a significantly positive correlation between total estradiol concentration and unbound estradiol concentration (r=0.95, p<0.005, y=64X+4.6). A positive correlation between unbound estradiol and bound estradiol concentration (r=0.94, p<0.005, y=0.08+0.014X) was also observed. On the other hand, there was a significant inverse relationship between unbound estradiol concentration and SHBG-testosterone ratio (r=-0.88, p<0.005, y=78-23X). These data indicate that total estradiol and testosterone and SHBG-testosterone ratio play an important role in the regulation system of unbound estradiol.