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Using specific radioimmunoassays, 17,20-dihydroxypregn-4-en-3-one and progesterone were measured in maternal utero-ovarian venous plasma from four sheep during parturition induced with dexamethasone and two sheep at spontaneous delivery. 17,20-Dinydroxypregn-4-en-3-one was also measured in maternal jugular venous and foetal posterior vena caval plasma from one dexamethasone-induced animal over the same period. Basal utero-ovarian venous levels of 17,20-dihydroxypregn-4-en-3-one were in the range 3– 7 ng/ml; at the time of the pre-labour fall in maternal progesterone, the concentration of 17,20-dihydroxypregn-4-en-3-one increased, reaching 17–76 ng/ml at delivery. Maternal levels fell rapidly after delivery. In contrast to foetal progesterone concentrations, which are low, foetal 17,20-dihydroxypregn-4-en-3-one levels were approximately the same as those in the maternal utero-ovarian vein. Maternal jugular venous levels in one animal were 50–80% of those in the utero-ovarian vein. These findings confirm earlier results obtained in vitro which indicated that exposure to glucocorticoid results in increased activity of a steroid 17-hydroxylase in the ovine foetal placenta.