During reproductive processes, prostaglandin (PG) E₂ (PGE₂) and PGF_2α play important roles in which they often exert opposite effects. At the time of recognition of pregnancy in vivo, PGF_2α is recognized as the luteolytic factor in ruminants and in most species including human, whereas PGE₂ may exert a luteoprotective action. We have previously demonstrated that recombinant interferon-tau (rIFN-τ), the embryonic signal responsible for recognition of pregnancy in ruminants, stimulated in vitro the production of PGE₂ and prostaglandin-endoperoxide synthase 2 (Ptgs2; also called cyclooxygenase-2) gene expression in both epithelial and stromal endometrial cells. Since PGE₂ is the major prostaglandin produced by stromal cells, the effect on Ptgs2 could explain the increase in PGE₂ output. At high concentrations, however, recombinant ovine (ro) IFN-τ acts on epithelial cells by changing the primary PG produced from PGF_2α to PGE₂. This change in the primary PG produced could be explained by a decrease in PGF synthase (PGFS) activity or an increase in PGE synthase activity, or by modulation of a putative PGE₂–9-ketoreductase, which converts PGE₂ into PGF_2α. Therefore, we have investigated the regulation of the mRNAs for PGFS and PGE₂-9-ketoreductase (9K-PGR), two enzymes that lead to the production of PGF_2α. Others have described 9K-PGR activity in uterus, ovaries, kidney, and liver of different species and have established that this enzyme could possess both 9K-PGR and 20α-hydroxysteroid dehydrogenase (20α-HSD) activity. Some have concluded that 9K-PGR and 20α-HSD are identical enzymes. Using primers sequences chosen from homologous nucleotide sequences of published rabbit 20α-HSD/9K-PGR and rat 20α-HSD cDNAs, a 317-base pair (bp) fragment was amplified by reverse transcription-polymerase chain reaction (RT-PCR), cloned, and sequenced. Homologies of 83% and 78% were found with rabbit and rat 20α-HSD, respectively. The presence of 20α-HSD/9K-PGR and prostaglandin F synthase (PGFS) mRNA expression was studied semiquantitatively in cultured epithelial cells using RT-PCR. Stimulation of cells with roIFN-t resulted in a biphasic response, an inhibition of PGF_2α production at low dose (1 ng/ml) and a stimulation of PGE₂ at high dose (10 μg/ml). The increase of PGE₂ was accompanied by reduced 9K-PGR and PGFS mRNA gene expression. The effect of oxytocin (OT) was also studied, and the presence of OT had no effect on either 9K-PGR or PGFS gene expression. The 20α-HSD/9K-PGR transcript was also detected in other bovine tissues at different intensity (liver > kidney > testis > ovaries). We believe that the 9K-PGR and PGFS can be key enzymes in the regulation of specific PGs in the endometrium during the periimplantation period.