Changes in the protein composition of the embryo's environment during early development were studied by analysis of proteins synthesized and secreted by oviducal and uterine explants on Days 1–6 of pregnancy. Although secretions from ampullar and isthmic oviduct and uterus contained many proteins in common, each area also produced its own characteristic proteins. In the uterus, changes in the secretion pattern were found during the peri-implantation period, including both increases and decreases in particular proteins which appear to be dependent on the presence of embryos. Embryo-induced effects on uterine secretion began between 09:00 h of Day 4 and 09:00 h of Day 5. Oviducal secretions exhibited many of the embryo-dependent proteins found in the uterus, but the expression of these proteins did not appear to be influenced by the presence of embryos on Day 1 or Day 3. The characteristic pattern of secreted protein expression by each portion of the reproductive tract may reflect the specialization of each area for certain developmental events.