In previous study, I have reported that ⊿(4)-3-ketosteroid 5α-oxidoreductase activity in the rat diencephalon was decreased by diphenylhydantoin, imipramine and chlorpromazine administered intraperitoneally. At the same time, it was observed that 5α-androstan-3α, 17β-diol formation was decreased. In this paper, I report on the occurrence of 3α-hydroxy-steroid dehydrogenase and the effect of the central acting drugs on the enzyme activity. The results were as follows. 1) 0.2 μ Ci of 4.(14)C-dihydrotestosterone (specific activity 50.6mCi/mM), 1 ml of rat diencephalon homogenate (about 8 mg protein) and NADPH (95μM/L) were mixed together and the total volume was adjusted to 2.5ml with phosphate buffer (pH 7.0). The reaction mixture was incubated at 38℃ for 120min. with continual shaking, the reaction contents were exposed to air. A sample of enzyme preparation placed in a boiling water bath for 5min. was used as blank. Identification of the metabolite was performed by TLC and GLC. 5α-androstan-3α, 17β-diol was confirmed as principal metabolite. 2) The occurrance of 3α-hydroxysteroid dehydrogenase was confirmed in rat diencephalon. Optimal pH of the enzyme was around 7.0 and apparent Km values was estimated as 5.9 x 10(-6)M. 3) Male Wistar rats weighing about 100g were divided into four groups. Each group received chlorpromazine HCl (0.4mg/0.1ml/100g body weight), imipramine HCl (0.1mg/0.1ml/100g body weight), sodium diphenylhydantoin (0.5mg/0.1ml/100g body weight) or normal saline (0.1ml/100g body weight) intraperitoneally for 14 consecutive days respectively. 24 hours following the last administration of the drug, the rats were killed by decapitation. Diencephalon was removed and homogenated with 9 volumes of phosphate buffer (pH 7.0). Homogenated were incubation with dihydrotestosterone and NADPH for the estimation of the 3α-hydroxysteroid dehydrogenase activity. The enzyme activity was not affected by these drugs.