The presence of prolactin, in quantities comparable with the adult, has been confirmed in the plasma of the fetal sheep a few days before term (140 and 144 days) but at 101 and 105 days only traces were occasionally found despite the presence of considerable quantities in the fetal pituitary. Prolactin was undetectable in the plasma of the young fetuses after administration of reserpine (2.5 mg i.v.) and the same dose of reserpine did not raise the plasma levels in the older fetuses. The only cardiovascular effect was a transient slight bradycardia. In all animals, however, the maternal plasma prolactin rose markedly after a short delay. A similar rise was seen in anoestrous ewes given the same dose intravenously. It is concluded (a) that reserpine is not an adequate stimulus for prolactin release in the sheep fetus although it is in the adult ewe and (b) reserpine rapidly crosses the ovine placenta from fetus to mother.