Androgen levels in early neonatal urine were measured by GC-MS using trideuterated steroids at C-19 as internal standards. The deuterated steroid is not only stable in the process of analysis but also behaves chemically the same as the corresponding steroid to be measured so that the reliability in the method of analysis is satisfactory. The following results were obtained by this method: Androsterone levels in early neonatal urines were significantly higher in male than in female on the 1st, 3rd and 5th day, though the levels fell significantly from the 1st to 3rd day. Testosterone values were also higher in male than in female on the 1st, 3rd and 5th day. No sex difference in dehydroepiandrosterone or dihydrotestosterone was observed. The sex difference of the androgen levels indicated a testicular steroidogenesis in male neonates.