In samples from twenty chronically cannulated ovine fetuses the plasma immunoreactive adrenocorticotrophin (ACTH) concentrations were 12.5 +/- 3.2(8), 15.2 +/- 4.1(9) and 21.2 +/- 5.6(8) pg/ml at periods, prior to parturition, of -30 to -35, -25 to -29 and -20 to -24 days respectively. Values are mean +/- SEM (number of samples). These values were not significantly different from each other but were significantly lower (P less than 0.02) than values in the next two age groups -36.0 +/- 4.9(7) pg/ml at -19 to -15 days, and 39.6 +/- 6.6(11) pg/ml at -14 to -9 days. A further significant increase (P less than 0.05) occurred in the -8 to -3 day period, ACTH being 53.9 +/- 5.4(12) pg/ml. On day of delivery two samples had values of 325 and 360 pg/ml. A single injection, intravenously of 1.0 microgram ovine corticotrophin-releasing factor (O-CRF), caused a significant increase in fetal plasma ACTH concentrations in fetuses of -6 to -23 days prior to delivery but not in fetuses -24 to -35 days prior to parturition. The maximum values of ACTH after O-CRF were significantly greater in fetuses -2 to 0 days prior to parturition than in younger fetuses (P less than 0.01). In 6 experiments in 4 fetuses (parturition -1 to -13 days) the effect of 1.0 microgram O-CRF persisted for at least 2.5 h. The results support the hypothesis that the pituitary release of ACTH changes sensitivity to hypothalamic O-CRF at least twice during the last fifth of gestation; an increasing sensitivity is seen as term approaches.