Uterine secretory proteins protect the uterus and conceptuses against infection, facilitate implantation, control cellular damage resulting from implantation, and supply embryos with nutrients. The early conceptus of the European polecat (Mustela putorius) grows and develops free in the uterus until implanting at about 12 days after mating. Using a proteomics approach we found that the proteins appearing in the uterus leading up to and including the time of implantation changed dramatically with time. Several of the proteins identified have been found in pregnant uteri of other placental mammals, such as α1-antitrypsin, serum albumin, lactoferrin, cathepsin L1, uteroferrin, and ectonucleotide pyrophosphatase. The broad-spectrum proteinase inhibitor α2-macroglobulin rose from relatively low abundance initially to dominate the protein profile by the time of implantation. Its functions may be to limit damage caused by the release of proteinases during implantation, and to control other processes around the site of implantation. Lipocalin-1 (also known as tear lipocalin) has not previously been recorded as a uterine secretion in pregnancy, and also increased substantially in concentration. If polecat lipocalin-1 has similar biochemical properties to the human form, then it may have a combined function in transporting or scavenging lipids, and antimicrobial activities. The changes in the uterine secretory proteome of Euroepan polecats may be similar in those species of mustelid that engage in embryonic diapause, but possibly only following reactivation of the embryo.