An enzyme catalyzing the reduction of metyrapone, a diagnostic drug with a ketone group, was partially purified from liver microsomes of male rats. The partially purified metyrapone reductase had no ability to reduce acetohexamide, an oral antidiabetic drug with a ketone group, even though both metyrapone and acetohexamide are reduced in liver microsomes of male rats. These results clearly indicate that the reduction of these two drugs can be catalyzed by different enzymes. The partially purified metyrapone reductase was found to reduce aldehydes, ketones and menadione. The substrate specificities were in fair agreement with those of carbonyl reductase. However, the partially purified enzyme was strongly inhibited by inhibitors of aldehyde reductase, such as barbital, phenobarbital and sodium valproate.