Fetal and maternal plasma concentrations of androstenedione, testosterone, and estradiol were measured in chronically catheterized pregnant sheep during late gestation, spontaneous term delivery, and premature delivery induced by the infusion of low doses of synthetic ACTH-(1-24) (ACTH) to the fetus at 120-130 days gestation. RIAs employing two different antisera directed at different sites of each molecule were used in combination with Celite column chromatography to ensure the specificity of the hormone measurements. Fetal plasma concentrations of all three hormones were greater than the respective maternal concentration at all periods investigated, except immediately before ACTH-induced delivery when fetal testosterone and estradiol concentrations were lower. These observations further support the hypothesis that delivery in the sheep is associated with the induction of placental 17 alpha-hydroxylase and 17,20-lyase by the increased conversion of progesterone to estrogens. The higher values of androstenedione than of testosterone also support the view that estrone is produced in larger quantities than estradiol and that estrone and estrone sulfate are good endocrine markers of the progress of the endocrine changes occurring immediately before parturition in the sheep.