It is considered that plasma-free testosterone is a bioactive androgen in blood and is more reflective in androgenicity than plasma total testosterone. We measured plasma-free testosterone by dialyzable method in normal menstrual females, pregnant females, post menopausal females and vulvar dystrophy. The values of plasma total testosterone (T), fractional free testosterone (%FT) and plasma-free testosterone (free T) were 0.61 +/- 0.27 (mean +/- SD)ng/ml, 1.19 +/- 0.34% and 0.61 +/- 0.20 ng/kl in the follicular phase of normal menstrual females (n = 25), and 0.57 +/- 0.23 ng/ml, 1.29 +/- 0.46% and 0.56 +/- 0.35 ng/dl in the luteal phase of normal menstrual females (n = 24), respectively. The values in both phases showed no differences from each other. In post menopausal females (n = 18), the concentration of T (0.36 +/- 0.27 ng/ml) and free T (0.52 +/- 0.12 ng/dl) was significantly lower (T: P less than 0.01, free T: P less than 0.05) than that in normal menstrual females. However, %FT in post menopausal females (1.37 +/- 0.12%) was not different as compared with that in normal menstrual females. In pregnant females (n = 2) of second and third trimester, T (0.53 +/- 0.07 ng/ml) was not different as compared with that in normal menstrual females, but %FT (0.67 +/- 0.45%) and free T (0.33 +/- 0.23 ng/dl) were significantly lower (P less than 0.05) than in normal menstrual females. In vulvar dystrophy, lichen sclerosus (n = 19) and hyperplastic dystrophy without atypia (n = 12) were measured.(ABSTRACT TRUNCATED AT 250 WORDS)