Abstract: 17α-Methyltestosterone (Ⅰ) and related steroids were studied for their anabolic and androgenic activities in castrated rats and mice. For rats the levator ani weight was taken as the criterion of anabolic activity, and the weight of the seminal vesicles served as a measure of androgenic activity. For mice the same parameters were used and, besides, the kidney weight was employed as an additional measure of anabolic activity. Results indicated that the anabolic and androgenic activities of 17α-methyl-(5α)-androstane-17β-ol-3-one (Ⅱ) or 17α-methyl-(5α)-androstane-3β, 17β-diol (Ⅲ), 4,5-hydro-genated analogues of (Ⅰ), at a dosage of 30 mg/kg by garage in castrated rats, were not significantly different from those of Ⅰ. However, when the doses were reduced to 12.5 mg/kg, the anabolic activity of Ⅱ in castrated mice was much greater than that of Ⅰ, while the androgenic activity of the two compounds was approximately equal. The anabolic and androgenic activities.of Ⅲ were weaker as compared with those of Ⅰ.The compound 17α-methyl-(5α)-androstane-17β-ol (Ⅳ) (3-deoxy-analog of Ⅰ) at a daily dosage of 30 mg/kg by gavage in castrated rats was shown to be more active on the levator ani and the seminal vesicles than Ⅰ. In castrated mice, Ⅳ caused similar anabolic effects as Ⅰ in doses of 12.5 mg/kg, while its androgenic potency was only one third of that of Ⅰ. Castrated rats and mice were treated with 5α-androstane-3-ol and 5α-androstane-3-one (17-deoxy analogue of testosterone) by gavage or by injection at a dosage of 30 and 40 mg/kg. Neither anabolic nor androgenic activities were observed. From the above results, it may be concluded that removal of the oxygen atom from C₃ and hydrogenation at C₄ and C₅ in 17α-methyltestosterone molecule may increase anabolic and/or decrease androgenic activity, while removal of the oxygen from C₁₇ of testosterone results in loss of both anabolic and androgenic activities. In view of the increase in anabolic activity by removal of the oxygen at C₃, synthesis of and pharmacological studies on a series of 3-deoxy analogues of testosterone and cortisone are in progress and will be reported in due course.