Structural and functional development of the corpus luteum (CL) involves tissue remodeling, angiogenesis, lipid metabolism, and steroid production. The peroxisome proliferator-activated receptors (PPARs) have been shown to play a role in these as well as in a multitude of other cellular processes. To examine the expression of mRNA corresponding to the PPAR family members (α, δ, and γ) in luteal tissue, ovaries were collected from gonadotropin-treated, immature rats on Days 1, 4, 8, and 14 of pseudopregnancy and from adult, cycling animals on each day of the estrous cycle. Ovaries were processed for in situ hybridization or RNA isolation for analysis by RNase protection assay. The expression of PPARγ mRNA was abundant in granulosa cells of developing follicles during both pseudopregnancy and the estrous cycle and was low to undetectable in CL from pseudopregnant rats. However, luteal tissue in cycling animals, especially CL remaining from previous cycles, had high levels of PPARγ mRNA. The PPARα mRNA was localized mainly in the theca and stroma, and PPARδ mRNA was expressed throughout the ovary. Levels of mRNA for PPARγ decreased between Days 1 and 4 of pseudopregnancy, and PPARα mRNA levels were lower on the day of estrus compared to pro- and metestrus (P < 0.05). The PPARδ mRNA levels remained steady throughout the estrous cycle and pseudopregnancy. These data illustrate a difference in the luteal expression of mRNA for PPARγ between the adult, cycling rat and the immature, gonadotropin-treated rat. This differential pattern of expression may be related to the difference in timing of the preovulatory prolactin surge, because the gonadotropin-primed animals would not experience a prolactin surge coincident with the LH surge, as occurs in adult, cycling animals. Additionally, the expression pattern of PPARδ mRNA indicates that it may be involved in cellular functions involved with maintaining basal ovarian function, whereas PPARα may play a role in lipid metabolism in the theca and stroma.