A number of 17 beta-(N-alkyl/arylformamido)- and 17 beta-[(N-alkyl/aryl)alkyl/arylamido]-3-oxo-4-aza-5 alpha-steroids were prepared from 17 beta-hydroxy-4-azasteroids and evaluated as inhibitors of human 5 alpha-reductase and antagonists of the androgen receptor. Jones' oxidation of 17 beta-hydroxy compounds gave the 17-keto-4-azasteroids, which were treated with amines and NaBH(OAc)3/NaBH3CN to give 17 beta-(N-alkyl/arylamino)-4-azasteroids 10-27. Alternatively, the above-indicated compounds were prepared from amines and 17-keto-4-azasteroids to form imines, which were then reduced with NaBH4. Formylation of amines 10-27 gave 17 beta-(N-alkylformamides) 28-41; however, acylation afforded 17 beta-[(N-alkyl/aryl)alkyl/arylamides] 42-53. In comparison to N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 beta-carboxamide (4-MA; IC50 = 4.15 nM), 17 beta-(N-alkylformamido)-4-azasteroids were potent inhibitors of human type I 5 alpha-reductase, IC50 values of compounds 29, 30, 36, and 37 being measured as 3.05, 0.91, 2.19, and 2.35 nM, respectively. The structure-activity relationships suggest that the type I enzyme has preference for N-substituted straight alkyl side chains of four to five carbon atoms. On the other hand, formamides 32 (N-heptyl) and 33 (N-octyl), in addition to inhibiting the type I enzyme (IC50s = 9.57 and 16.9 nM, respectively), showed also strong inhibitory activity (IC50s = 14.0 and 18.4 nM, respectively) for human type II 5 alpha-reductase, in comparison to N-(1',1'-dimethylethyl)-3-oxo-4-aza-5 alpha-androst-1-ene-17 beta-carboxamide (MK-906; IC50 = 4.53 nM). Other compounds in this series showed moderate activities (IC50 > 100 nM) on the type II enzyme. 17 beta-[(N-Alkyl/aryl)alkyl/arylamides] 45, 46, 48, and 51 exhibited highly potent inhibitory activity for human type I 5 alpha-reductase with IC50s of 1.77, 2.42, 2.93, and 5.44 nM, respectively, while moderate to no effect was observed on the type II enzyme (100 < IC50s < 1000 nM), except for compound 48 (IC50 = 3.75 nM). In another substitution pattern, N-aryl/alkylamides were studied; an electron-donating group increased the potency of compound 51, whereas an electron-withdrawing group decreased the potency of compounds 52 and 53 compared to parent compound 50. In addition to their 5 alpha-reductase activities, 17 beta-(N-alkylformamides) were also studied for their inhibitory activities on dihydrotestosterone (DHT)-stimulated proliferation of androgen-sensitive Shionogi mouse mammary carcinoma cells (clone SEM-107).(ABSTRACT TRUNCATED AT 400 WORDS)