Pregnane steroids have sedative and neuroprotective effects on the brain as a result of interactions with the steroid-binding site of the GABAA receptor. To determine whether the fetal brain is able to synthesize pregnane steroids de novo from cholesterol, we measured the expression of cytochrome P450 side-chain cleavage (P450scc) and 5alpha-reductase type II (5alphaRII) enzymes in fetal sheep from 72 to 144 d gestation (term approximately 147 d) and in newborn lambs at 3 and 19-26 d of age. Both P450scc and 5alphaRII expression was detectable by 90 d gestation in the major regions of the brain and also in the adrenal glands. Expression increased with advancing gestation and was either maintained at fetal levels or increased further after birth. In contrast, the relatively high content (200-400 pmol/g) of allopregnanolone (5alpha-pregnan-3alpha-ol-20-one), a major sedative 5alpha-pregnane steroid, present throughout the brain from 90 d gestation to term, was reduced significantly (<50 pmol/g) immediately after birth. These results suggest that although the perinatal brain has the enzymes potentially to synthesize pregnane steroids de novo from cholesterol, either the placenta is a major source of these steroids to the brain or other factors associated with intrauterine life may be responsible for high levels of allopregnanolone production in the fetal brain until birth.