In humans, two isozymes of 5α-reductase (5AR) catalyze the conversion of testosterone (T) to the more potent androgen dihydrotestosterone (DHT). Male pseudohermaphrodites with 5AR deficiency have mutations in type 2 5AR and specific phenotypes, while type 1 5AR is normal and contributes to residual plasma DHT. The relative roles of human type 1 and 2 5AR in developmental physiology and pathophysiology of disorders like benign prostatic hyperplasia (BPH) are intensely researched. Existing 5AR inhibitors include finasteride and epristeride. We report novel 6-azaandrost-4-en-3-ones that are potent inhibitors of both human 5ARs, with exceptional picomolar potency against the type 2 enzyme. Synthesis of the 6-azaandrost-4-en-3-one nucleus followed Lettré's strategy to introduce nitrogen at the 6-position, avoiding elimination issues via silica gel-induced cyclization. In vitro assays using microsomes from baculovirus-infected SF9 cells expressing human type 1 or 2 5AR showed potency versus both isozymes dominated by lipophilicity of the C-17 substituent (e.g., adamantyl substitution enhanced type 1 potency). In vivo evaluation in a chronic castrated rat model showed the most active compounds equivalent to finasteride in inhibiting T-stimulated prostate growth. Promising compounds (e.g., 18, 20, 21) had half-lives of 4.8-12 h in dogs, with compound 18 showing 80% oral bioavailability. These potent dual 5AR inhibitors may help define the relative roles of human type 1 and 2 5AR in pathophysiology of androgen-dependent diseases.