Fetal glucocorticoid-induced premature labor in sheep is an established model of premature labor. However, the pathways by which fetal cortisol triggers subsequent maternal endocrine changes, including enhanced PG synthesis, leading to labor are unclear. The current study was undertaken to determine whether cortisol administration to adrenalectomized fetuses to clamp fetal cortisol at levels present early in the late gestation rise, which are inadequate to produce labor, can stimulate placental, myometrial, and endometrial prostaglandin G/H synthase 2 mRNA and protein expression. At 109--13 d gestation, fetal sheep adrenals were removed (n = 8), or sham surgery was performed (n = 4). From d 6 postadrenalectomy, maternal and fetal plasma cortisol were determined daily by RIA. From d 7 postadrenalectomy, cortisol (4 micro/min) was continuously infused iv to four adrenalectomized fetuses. Endometrium, myometrium, and placentome were collected from all three groups of ewes (n = 4 for each group), and total RNA and proteins were extracted from each intrauterine tissue and analyzed by Northern and Western for prostaglandin G/H synthase 2 mRNA and protein. P45017 alpha hydroxylase mRNA was analyzed in the placentome by Northern blot. Data were analyzed by ANOVA. Plasma cortisol levels remained low in sham-operated and adrenalectomized fetus, whereas during cortisol infusion to adrenalectomized and cortisol-treated fetuses, plasma cortisol increased to the late gestation level. After adrenalectomy, prostaglandin G/H synthase 2 did not change in any tissue studied. Fetal plasma cortisol replacement to late gestation levels increased prostaglandin G/H synthase 2 to levels similar to term levels in all three tissues. PGHS1 mRNA and protein did not change in any group studied. There was a minimal increase in P45017 alpha hydroxylase mRNA in the placentome in the adrenalectomized and cortisol-treated group. Cortisol- induced labor further increased P45017 alpha hydroxylase mRNA in the placentome compared with that in adrenalectomized and cortisol-treated animals. These data provide evidence for in vivo cortisol up-regulation of prostaglandin G/H synthase 2, but not PGHS1, in late gestation in the ovine placentome, myometrium, and endometrium. As stimulation of the estrogen biosynthetic pathway was minimal in the adrenalectomized and cortisol-treated group, these data provide support for the concept that cortisol has a direct effect on prostaglandin G/H synthase 2 expression in addition to its classical indirect pathway on prostaglandin G/H synthase 2 as a result of estrogen synthesis.