A determination of steroid dehydrogenases utilizing cryostat technic has been accepted since Wattenberg reported. However, difficulties were also encountered on sectioning technic or standarization of enzymatic conditioning. The present study was design to establish new device of "en block" staining without cryostat technic. Determination of placental enzyme of 3β-ol- steroid dehydrogenase, 11β-hydroxysteroid dehydrogenase, 17β-hydroxysteroid dehydrogenase and 20β-hydroxysteroid dehydrogenase were performed by new technic and gestational changes in dehydrogenase were also studied in this paper. 1). 3β-ol-steroid dehydrogenase Marked activity of 3β-ol-steroid dehydrogenase was observed in syncytial trophoblast throghout a pregnancy. 2). 11β-hydroxysteroid dehydrogenase The lowest activity among dehydrogenase messured was noted in 11β-hydroxysteroid dehydrogenase, which was localized in stroma of villi. 3). 17β-hydroxysteroid dehydrogenase and 20β-hydroxysteroid dehydrogenase. 17β-hydroxysteroid dehydrogenase and 20β-hydroxysteroid dehydrogenase was found to be localized in stroma and vessel wall showing strong activity. It may be concluded the 3β-ol-steroid dehydrogenase is a essential factor for maintainance of pregnancy in connection with progesterone biosynthesis, while 17β-hydroxysteroid dehydrogenase and 20β-hydroxysteroid dehydrogenase may act on platental function in steroid regulation after middle stage of pregnancy.