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Vascular endothelial growth factor (VEGF) and its specific receptors FLT-1 and FLK-1 represent an important ligand-receptor system involved in angiogenesis and permeability. These factors are supposed to be influenced by ovarian steroids involved in developmental changes in female reproductive tissue as oviduct and uterus. The aims of this study were to assess the expression of VEGF and its receptor mRNAs during the early implantation period in porcine endometrium using real-time RT-PCR. Furthermore, effects of estradiolbenzoate (EB) and progesterone (P) on endometrium of ovariectomized (ovx) pigs were examined by RT-PCR and immunohistochemistry. A complete VEGF system was found in endometrial tissue using RT-PCR detecting the main VEGF isoform 188 aa, FLT-1 and FLK-1. A significant upregulation of the mRNAs of VEGF and its receptors was observed in the endometrium during the peri-implantation when compared with the pre-implantation period. Regarding endometrium of non-pregnant ovx-pigs an application of P led to elevated transcript levels of VEGF whereas mRNA-expression was reduced after EB treatment compared to non-treated ovx-animals. When pigs were administrated EB and P simultanously, a decrease in VEGF mRNA concentration was recorded. For FLT-1, none of the steroids increased mRNA expression compared to the ovx-group. Analysis of FLK-1 receptor mRNA demonstrated that only after EB + P treatment mRNA-expression was stimulated but stayed unchanged after P and EB when compared with the ovx-group. Immunohistochemistry revealed FLK-1 and VEGF proteins in glandular and luminal epithelia of the endometrium with emphasized staining after P and P + EB treatment of ovx-pigs. Summarized, altered VEGF and FLK-1 expression during the implantation period as well as under steroid hormones suggest this growth factor as a potent regulator of hyperpermeability supporting the angiogenic process in porcine endometrium.