1) The estradiol-17β dehydrogenase was extracted from human abdominal rectus muscle as a non-target organ, various kinds of human uterus and term placenta as the target organ of sex hormones and its activities were assayed by the Langer and Engel's method.
2) The activity of the estradiol-17β dehydrogenase was neglegible in abdominal rectus muscle, markedly low in senile atrophic uterus, moderately low in normal uterus, moderately high in fibroid or pregnant uterus and markedly high in the term placenta in both estradiol-17β dehydrogenation and estrone hydrogenation reaction. These results suggest that this enzyme is closely related with the uterus-sensitivity to sex hormones.