Freshly isolated rat hepatocytes metabolized the nitrogen-containing steroid N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androst-1-ene-17 beta- carboxamide (I) to six products analyzed by reverse phase HPLC. The major metabolite, which could account for greater than 50% of the total I-related material, exhibited chromatographic, NMR, and mass spectral properties identical to those of the authentic 4-carbinolamide of I. Thus, the major biotransformation pathway in hepatocytes was hydroxylation of the N-methyl group of I to form a stable carbinolamide intermediate of N-demethylation. Desmethyl-I was observed as a minor metabolite. Another metabolite which accounted for approximately 10% of the total I-related material had chromatographic and spectral properties identical to those of the authentic monoethyl analog of I. The other three metabolites were formed in variable quantities and were unstable when isolated. Mass spectral data suggested that one metabolite was the carbinolamide intermediate of N-deethylation. Treatment of rats with phenobarbital or dexamethasone increased the formation of the monoethyl metabolite of I in hepatocytes but had no effect upon the formation of the 4-carbinolamide metabolite. Rat hepatic microsomes catalyzed the NADPH-dependent metabolism of I to the same metabolites in the same relative amounts as observed with intact hepatocytes. Studies with alternative substrates and inhibitors demonstrated that microsomal cytochrome P-450 was responsible for the metabolism of I. Dog and human hepatic microsomes metabolized I to the same products as rat hepatic microsomes, but monoethyl I was the major metabolite.