B-Nor-4-aza-5a-androstane derivative 1 was synthesized. The compound 1 showed potent inhibition activity against testosterone 5a-reductase.Testosterone 5a-reductase plays an important role in the pathology of benign prostatic hypertrophy. It converts testosterone in the prostate to 5a-dihydrotestosterone, whose accumulation is known to cause hypertrophy of the prostate.A series of 4-aza-steroid compounds have already been synthesized, and they were found to have 5a-reductase inhibition activity. Among them, N-(1,1-dimethylethyl)-3-oxo-4-aza-5a-androst-1-ene-17b-carboxamide, MK-906, was recently launched as a drug for benign prostatic hypertrophy. These compounds have a nitrogen at the C-4 position and a backbone structure similar to that of testosterone. In relation to the structure-activity relationship we were interested in modifying the backbone structure, in particular the B-ring. Namely, we focused attention on synthesizing a B-nor-4-aza-5a-androstane derivative. The conformation of B-nor-4-aza-5a-androstane, having a five-membered B ring, is not much different from that of 4-aza-5a-androstane based on molecular model inspection. In addition, we have already disclosed that the 4-aza-androstane derivative 2, having a 17b-benzhydrylcarbamoyl moiety, has highly potent inhibition activity against testosterone 5a-reductase. Therefore, the B-nor-4-aza-5a-androstane derivative 1 with a benzhydrylcarbamoyl moiety at the C-17 position was selected as a target compound. In this report we describe a synthesis of 1 and its 5a-reductase inhibition activity.