Plasma concentrations of 17OH-progesterone were determined in 60 normal newborn infants aged between 3 and 36 hours. Mean levels decreased rapidly during this time after removal of the placental contribution of this steroid. A further 70 normal infants, studied between ages 2 and 7 days, showed a mean plasma 17OH-progesterone concentration of 3.5 nmol/1 (1.2 ng/ml). By comparison, plasma concentrations in untreated infants with congenital adrenal hyperplasia were markedly raised. At 36 hours of age, there was an obvious difference between plasma levels of this steroid in normal and affected infants. Determination of plasma 17OH-progesterone concentrations are valuable in the evaluation of disorders of sexual differentiation and electrolyte balance in newborn infants, provided due care is given to the timing of sample collections.