In this article we show that rabbit endometrial cells express leptin receptor and that human leptin triggers phosphorylation of signal transducer and activator of transcription 3 and up-regulates the expression of interleukin- 1 receptor type I as was previously found in human endometrial cells. Interestingly, leptin also upregulates the secretion of leukemia inhibitory factor and expression of its receptor by rabbit endometrial cells. Analysis of a structural model of the leptin-leptin receptor complex suggested that helices I and III of the human leptin structure were likely sites of interaction with the cytokine binding domain of leptin receptor. Accordingly, we synthesized a peptide (LPA-2) comprising helix III (residues 70-95) and investigated its ability to inhibit leptin receptor function. The effects of LPA-2 were assayed in rabbit endometrial cells, and an antileptin receptor antibody and a scrambled version of LPA-2 were used as positive and negative controls, respectively. LPA-2 binds specifically and with high affinity (Ki ~ 0.6 x 10-10 M) to leptin receptor and is a potent inhibitor of its functions in rabbit endometrial cells. Because leukemia inhibitory factor and interleukin- 1 have been implicated in embryo implantation, our results raise the possibility that the LPA-2-induced inhibition of leptin receptor may be exploited to study the actions of leptin in endometrium and in other tissues under conditions characterized by abnormal leptin production.