This study investigates the development of the feedback relationship between cortisol and ACTH in ovine fetuses at 117-119 (n = 5), 130-134 (n = 4) and 140-143 (n = 6) days of gestation. During the last 2 h of a 24 h cortisol (100 micrograms h-1) or saline (0.19 mL h-1) infusion, the fetal ACTH response to bolus oCRH injection (10 micrograms) was tested after the collection of basal ACTH and cortisol samples. Basal ACTH concentrations in un-infused or saline-infused fetuses were observed to progressively increase after 117-119 days of gestation. In contrast, a prepartum increase in cortisol concentrations could not be detected until after 130-133 days. Cortisol infusion significantly inhibited basal ACTH values at 130-133 and 140-143 but not at 117-119 days. In the two youngest age groups, cortisol infusion significantly inhibited the fetal ACTH response to oCRH. At 140-143 days, ACTH values after oCRH injection, were elevated to a similar extent during the saline or cortisol infusion. However, at 140-143 days there was evidence to suggest that the ACTH response to oCRH administered during the saline infusion was blunted when compared with earlier stages of gestation. This study suggests that the circulating, basal concentrations of both ACTH and cortisol increase during the last fifth of gestation in the ovine fetus. The exact nature of the prepartum feedback relationship that develops between these two hormones remains to be fully elucidated.