Urinary steroids from infants one to nine months of age have been analysed by gas chromatography and combined gas chromatography-mass spectrometry. Open tubular capillary columns were used for the gas chromatographic analyses. These greatly improved the resolution of steroid methyloxime-trimethylsilyl ethers over that obtained with packed columns. The major 3β-hydroxy-Δ5 steroids found in the younger infants were 3β,16α-dihydroxy-5-androsten-17-one, 3β,17β-dihydroxy-5-androsten-16-one, 5-androstene-3β,16α,17β-triol and 3β,16α-dihydroxy-5-pregnen-20-one. In the older infants 5-pregnene-3β,20α-diol and 5-pregnene-3β,20β-diol were the predominant 3β-hydroxy-Δ5 steroids. However, there was a general decrease in the amount of 3β-hydroxy-Δ5 steroids excreted relative to the total steroid excretion. Cortisol metabolites were found in urine from infants of all ages. The principal metabolite was tetrahydrocortisone the excretion of which varied between 100 and 700 μg/24 h. 5β-pregnane-3α,l7α,20α-triol (pregnanetriol) was assayed in each urine sample but only became quantitatively significant after the fourth month of life (20-120 μg/24 h in the later samples).