This study investigated the effects of repeated short term (2-h) intrafetal infusions of prostaglandin E2 (PGE2) on ACTH and cortisol release in fetal sheep during late gestation (119-144 days). We compared the effects of administration of PGE2 (2 micrograms/min) into the fetal carotid artery or jugular vein. PGE2 infusion significantly (P < 0.001) increased fetal plasma immunoreactive (ir-) ACTH and cortisol concentrations regardless of the vessel used for administration. Saline infusion did not alter the concentrations of ir-ACTH or cortisol for the duration of the experiment. To compare the responses of fetal ir-ACTH and cortisol to repeated intracarotid infusions of PGE2, the hormone data were grouped into five gestational age ranges (119-125, 126-130, 131-135, 136-140, and 141-145 days). Fetal ir-ACTH was stimulated by PGE2 infusion at all gestational ages studied; the greatest response was achieved at the earliest gestational age range, 119-125 days. PGE2 infusion preferentially stimulated the release of low mol wt ACTH [ACTH-(1-39); 60 min from the start of infusion] at all gestational ages (P < 0.01), but basal low mol wt ACTH did not increase with gestational age until after 140 days. Cortisol concentrations were increased within 30 min of infusion at all gestational ages studied. These results suggest that PGE2 may play a role in maintaining elevated ir-ACTH concentrations in the face of high levels of cortisol in fetal sheep before parturition.