The neurotrophins are a family of secreted growth factors which are abundantly expressed in the central and peripheral nervous systems. They are mainly known for their roles in promoting the development, growth, function, and survival of neurons. Brain-derived neurotrophic factor (BDNF), a neurotrophin, induces its effects by binding to the tyrosine receptor kinase B (Trk B). Although mainly known for their roles in the nervous system, BDNF and Trk B have recently been identified in the uterus. BDNF expression has been demonstrated in rat uterus, and Trk B expression was found in the human ovary, and endometrium. However, comparative mammalian reproductive cycle-dependent characteristics of the expression of this receptor-ligand pair has not been previously described. Here, C57/bl6 mouse (n=17), and Wistar rat uterine horns (n=11), and human endometrium (n=10) were cycle staged by histology and subsequently stained for BDNF and Trk B by immunohistochemistry. Uterine cell types on each section were assessed for staining intensity using a three-point scale. BDNF and Trk B were expressed in all samples of mouse, rat, and human uteri. In all species, BDNF and Trk B staining was concentrated in the luminal and glandular epithelium of the endometrium, and to a lesser degree in the myometrium. Antibody specificity was assessed by Western blot using human recombinant BDNF and Trk B proteins. Preliminary results suggest a decrease in Trk B expression from proestrus through to diestrus in the uterine luminal and glandular epithelium of mice and rats, while Trk B expression in women appears to remain constant between cycle phases. BDNF expression seems to mirror the expression pattern of Trk B in the species examined. Localizing BDNF and Trk B in the endometrium of humans, mice, and rats demonstrates their conserved nature across several mammalian species. The uterine expression of this receptor-ligand pair suggests that BDNF and Trk B participate in some currently unknown aspect of reproductive physiology in species that menstruate and those that do not. This work is supported by NSERC.