The expression profiles of leukemia inhibitory factor (LIF), transforming growth factor β2 (TGFβ2), and transforming growth factor β2 receptor (TGFβ2R) were analyzed during the peri-implantation period in regularly menstruating, fertile bonnet monkeys and in animals in which endometrial nonreceptivity was induced by administering an antiprogestin, onapristone. Based on our previous experiences, a dose of 2.5 or 5 mg of onapristone was administered s.c. every third day during the menstrual cycle, because these dosages impair endometrial development without upsetting the normal gonadal endocrine profiles. Endometrial biopsy specimens were collected during the proliferative phase (estradiol levels about 200 pg/ml, n = 5) and peri-implantation period (Day 8 after midcycle peak in estradiol levels, n = 5) from normal ovulatory animals and during the peri-implantation period from onapristone-treated animals (n = 10). The biopsy specimens were processed to determine the expression patterns of LIF, TGFβ2, and TGFβ2R by immunohistochemical and reverse transcription-polymerase chain reaction (RT-PCR) methods. Levels of both protein and mRNA for LIF, TGFβ2, and TGFβ2R (analyzed by immunohistochemistry and RT-PCR, respectively) were greater in the endometrial samples collected during the peri-implantation period compared to samples collected during the proliferative phase in control animals. Treatment with either of the two doses (2.5 or 5 mg) of onapristone caused a significant (P < 0.05) down-regulation in the expression of LIF in the peri-implantation endometria. The endometrial expressions of TGFβ2 and TGFβ2R mRNAs were reduced significantly in animals treated with 5 mg of onapristone, but not in those treated with the lower dose. However, immunoreactive TGFβ2 and TGFβ2R proteins were significantly (P < 0.05) down-regulated in the endometrial samples from both the 2.5- and 5-mg-treated groups. The alterations observed in the expression patterns of LIF, TGFβ2, and TGFβ2R were specific, because the expression levels of epidermal growth factor receptor remained unaffected in the endometria from the treated groups. The present study demonstrates derangement in the expression profiles of LIF, TGFβ2, and TGFβ2R during the peri-implantation period in infertile bonnet monkeys. It may be hypothesized that TGFβ2 function is one of the early steps in the regulation of the progesterone-driven cascade of events leading to endometrial receptivity, and that any aberration in this step may adversely affect the subsequent molecular events (i.e., expression of LIF). These data also suggest that potential aberrations in the functional network of locally produced cytokines and growth factors even may occur in an endometrium exposed to the optimal peripheral hormonal levels.