The 17α and 17β epimers of androstenediol (3β,17α-(or 17β)-dihydroxyandrost-5-ene) are both present in umbilical cord plasma (average total concn 120 μg./100 ml.) and in the urine of infants and adults, but the ratio is approximately 9:1 in foetal blood and infants' urine, and 1:2 in adults' urine (Shackleton, Livingstone & Mitchell, 1968). The 17β-hydroxy epimer of androstenediol has been isolated from foetal tissue (although it was possibly not distinguished from the 17α epimer) after perfusion with DHA (3β-hydroxyandrost-5-en-17-one) and DHA sulphate (Bolté, Wiquist & Dicsfalusy, 1966). It has been suggested that in early life the ratio in favour of 17α-androstenediol is due partly to the preferential 16α-hydroxylation of the 17β epimer in foetal tissue together with its preferential aromatization in the placenta to form oestradiol (Shackleton et al. 1968). In support of this suggestion it has now been shown that there is a preferential 16α-hydroxylation of the 17 β epimer