Synthetic ovine corticotrophin-releasing factor (oCRF) was infused continuously into the jugular veins of six ovine fetuses for 5-11 days. Two fetuses receiving 0.1 and 1.0 μg oCRF/h from gestational days 134 and 135 respectively, lambed prematurely on days 141 and 140 respectively. Three out of four fetuses receiving oCRF at 2.4 μg/h, from 125 days of gestation, delivered spontaneously at 131, 131 and 136 days, whilst one died in utero at 132 days, Two fetuses receiving vehicle only or oCRF intra-amniotically, were born at 148 and 145 days respectively, whilst six fetuses chronically cannulated but not infused were born at 149.8 ± 2.1 (S.D.) days. In ewes lambing at term, maternal plasma progesterone concentrations were 41.4 ± 11.4 (S.E.M; n = 5), 28.8 ± 7.8 (n = 6), 17.1 ± 4.8 (n = 5) and 7.9 ± 1.1 (n = 4) nmol/l on 3, 2, 1 and 0 days respectively before the lambs were born. No such decrease in maternal plasma progesterone concentrations was seen in the oCRF-infused fetuses. Fetal plasma concentrations of immunoreactive ACTH were maintained above normal in oCRF-infused fetuses, but some desensitization to bolus oCRF injections occurred in these fetuses. Four of the five fetuses born prematurely were sufficiently mature to survive, being able to stand, breathe and suckle. It is concluded that continuous oCRF infusions into immature fetuses can accelerate maturation of a number of organs and systems culminating in the premature delivery of viable lambs.