Early pregnancy in ruminants, such as the sheep, is characterized by relatively extensive development of the conceptus before attachment to the endometrium. Between the period of blastocyst hatching and initial attachment, the uterus responds to signals from the conceptus and adapts to provide an environment that permits the establishment of pregnancy. We used large-format two-dimensional (2D) PAGE to analyze the dynamic changes in protein composition of uterine luminal fluid (ULF) during this stage of pregnancy, and we determined the contribution of each of the extraembryonic membranes and the endometrium to these changes. The majority of the more than 40 pregnancy-associated proteins in ULF at Day 17 were secreted by the conceptus. By 2D gel map comparison and Western blotting, we identified transferrin, secreted by the yolk sac from Day 15, and cytoplasmic actin, one of the most abundant proteins produced by the trophoblast at Day 17. Apolipoprotein A1 and aldose reductase, whose abundance were markedly increased in pregnancy, were identified by peptide microsequencing. Aldose reductase, an enzyme required for the conversion of glucose to fructose, was shown to be synthesized by the trophoblast, and its detection even before the formation of the placenta suggests that the synthesis of fructose may occur much earlier than previously reported.