Northern blot and in situ hybridization techniques have demonstrated a marked increase in mRNA encoding epidermal growth factor (EGF) in the endometrium of mares, coincident with the start of interdigitation between the allantochorion and endometrium during placentation. In the present study, the unusually high EGF expression in the epithelium of the endometrial glands was shown to be maintained until at least day 250 of gestation (term = 320-340 days) in mares carrying normal horse conceptuses. However, in mares carrying failing donkey-in-horse pregnancies created by embryo transfer, EGF expression was severely retarded in those areas of the endometrium that were heavily infiltrated with lymphocytes and which showed a failure of placental development. Specific receptors for EGF were also detected in tissue homogenates from pregnant mares using 125I-labelled human EGF. Binding was high in the fetal membranes (allantochorion), both before implantation (days 30-34) and in the fully developed placenta (days 150-250), and was equivalent to the level of binding to homogenates of adult liver and kidney. Binding was much reduced in endometrial homogenates before implantation and from non-pregnant mares but increased after implantation to reach values equivalent to those exhibited by the fetal membranes. Scatchard analysis of displacement curves indicated a single class of high-affinity binding sites in the fetal membranes and pregnant endometrium sampled at day 150 of pregnancy and chemical cross-linking of the receptor-125I-labelled EGF complexes in fetal membranes revealed two radiolabelled bands of 170 kDa and 150 kDa. A large excess of insulin-like growth factor I (IGF-I) failed to displace any labelled EGF from the tissue homogenates. The marked and sustained upregulation of endometrial EGF expression during pregnancy in mares, and the presence of EGF receptors in the fetal allantochorion and maternal endometrium, suggest a possible role for EGF in the marked growth of these two tissues during placentation in equids.