- Feeding conjugated linoleic acids (CLA) improves reproductive performance in dairy cows; however, the molecular mechanisms by which CLA improves reproduction are not well understood. Therefore, we evaluated whether the CLA isomers, trans-10, cis-12 CLA and cis-9, trans-11 CLA altered synthesis of steroidogenic hormones in bovine luteal cells by measuring concentrations of progesterone, PGE₂, and PGF₂α in conditioned medium and expression of genes involved in their synthesis. Confluent luteal cells from each of 4 cows were cultured in 0 μM (control) or 0.1 μM solutions of trans-10, cis-12 CLA and cis-9, trans-11 CLA in varying ratios (1:0, 0:1, 1:1, 2:1, 1:2, 5:1, 1:5, 9:1, or 1:9) for 48 h in the presence and absence of 1 μM of the adenylate cyclase activator forskolin. Independent of CLA isomer and ratio, CLA decreased, compared to control, hormone concentrations of prostaglandin F₂α (62.6 ± 10.5 vs. 50.4 ± 9.9 pg/mL; P[subscript Overall] = 0.003) and, in the absence of forskolin, prostaglandin E₂ (61.2 ± 11.3 vs. 36.1 ± 10.1 pg/mL; P < 0.001) in cultured luteal cells, while no effect was observed for progesterone (P[subscript Overall] = 0.94). Compared to control, CLA decreased relative levels of COX-2 mRNA, a rate limiting enzyme in prostaglandin synthesis, by 1.7 fold (P[subscript Overall] < 0.001) and 3 β-hydroxysteroid dehydrogenase mRNA, a rate limiting enzyme in progesterone synthesis, by 1.4 fold (P[subscript Overall] = 0.008). Relative levels of PGE synthase and PGE₂ 9-keto-reductase mRNA, both involved in prostaglandin synthesis, and steroid acute regulatory protein and cytochrome P450 side chain cleavage mRNA, both involved in progesterone synthesis, were not significantly altered by CLA. In conclusion, a potential mechanism by which trans-10, cis-12 CLA and cis-9, trans-11 CLA may improve reproductive performance in dairy cows, is by suppressing PGF₂α synthesis in luteal cells through attenuating COX-2 gene expression.