||Methyltestosterone has been incubated with rat liver preparations. The metabolite found in the largest concentration was more polar than 17alpha-methyltestosterone. It was radioactive when 17alpha-methyltestosterone-20-C14 was incubated. No double bond or ketone groups have been demonstrated on the molecule. The steroid was capable of forming acetates indicating it was an alcohol. Essentially no radioactivity was lost during incubation which implies that the methyl group at position 20 was not lost during metabolism. This metabolite moved different from 17alpha-methyltestosterone-3alph,17beta-diol during chromatography. By the process of elimination of the possible reduced compounds, the metabolite was tentatively identified as either 3alph or the 3beta isomer of 17alpha-methyltestosterone-diol. Other radioactive substances have been isolated when 17alpha-methyltestosterone-20-C14 was incubated. These substances have a polarity similar to that of the corticosteroids. Further identification has not been carried out on these metabolites. The enzyme or enzymes responsible for the reduction of ring A have been found in the supernatant solution from 100,000 x g. centrifugation. The only cofactor which has been demonstrated to be required for complete saturation of ring A was TPNH. Attempted purification of the enzyme has been tried by using ammonium sulfate fractionation, calcium phosphate gel absorption, ethanol fractionation at reduced temperatures, and a second ammonium sulfate fractionation. Although an increased quantity of steroid reduced per mg. of nitrogen incubated was found during purification, no evidence of a separation of the enzyme systems was indicated. The major isolated from all of the fractionation procedures by chromatography appeared to have the same characteristics. No evidence of an intermediate ketone or delta-4 or delta-5-3-ol was found. The enzyme was found to be fairly labile to a change of time, temperature, and pH. p-chloromercuribenzoate completely inhibited enzymatic activity. Approximately 50 per cent inhibition by KCN was found with the high concentrations used. Although a sulfhydryl group seemed to be involved, no activation or protection of the enzyme was found in the presence of 0.005N cysteine. A number of steroids have a delta-4-3-ketone grouping in ring A and having different groups on position 17 were found to be substrates of the enzyme. Testosterone was shown to be the most active substrate of those that were used. Some radioactivity was found in the dihydrotestosterone isolated when testosterone-4-C14 was incubated in the presence of dihydrotestosterone. However, its specific activity was about one-tenth of that of the major metabolite. Kinetic measurements of the enzyme fraction obtained at 50 per cent of ammonium sulfate saturation but not at 40 per cent shoe two temperature optima and indicate two pH optima. The calculated Km was 2.64 x 10[-5] M. By the use of tritiated water, it appears that approximately one-fourth of the hydrogen ions required for the reduction of the double bond and ketone group in ring A are donated by the medium at this stage of enzyme purification. Whether or not one or more enzymes are required for the saturation of the double bond and the ketone group on ring A is discussed.