Studies were designed to examine cellular mechanisms that regulate direction of release of uterine secretory products in swine during the period when chemical messages from blastocysts signal the maternal unit that pregnancy is establsihed (maternal pregnancy recognition) (Days 12 to 15 postestrus). Uterine endometrium taken from gilts during early pregnancy and diestrus was examined by light and electron microscopic and biochemical techniques to examine effects of pregnancy. Changes in morphology of glandular epithelial tight junctions and basement membranes in pregnant and nonpregnant gilts were not associated with maternal pregnancy recognition. Basement membrane thickness increased between Days 10 and 12 of gestation and the estrous cycle. Changes in endometrial collagenase-like activity, total collagen and free hydroxyproline, which may reflect modification of basement membrane structure, were not associated with maternal pregnancy recognition. Direction of release of uterine secretory products by the glandular epithelium did not change during the period studied; however, secretory activity decreased on Day 18 in nonpregnant gilts. Cellular mechanisms regulating direction of release of uterine secretory products in swine were not found to be associated with changes in epithelial cell ultrastructure. Data demonstrated blastocyst influence on uterine morphology and secretory activity. Studies were designed to examine placental uteroferrin (Uf) transport and its distribution in fetal pigs. Uteroferrin is the major placental iron transport protein in pigs. Peroxidase-antiperoxidase (PAP) localization of Uf demonstrated uptake by epithelial cells of chorioallantoic areolae and subsequent release into placental capillaries. Uteroferrin concentrations in umbilical vein blood were greater (84%) than in umbilical artery blood. Uteroferrin was localized in Day 75 fetal kidney tissue by the PAP technique, and its presence in fetal urine was demonstrated by acid phosphatase activity, two-dimensional polyacrylamide gel electrophoresis and immunodiffusion analysis. Isolated Day 75 fetal liver plasma membranes bound 125 I-Uf in a dose-dependent manner. Binding, determined by light and electron microscope autoradiography, was specific for reticuloendothelial cells lining liver sinusoids. Uptake of bound Uf was mediated by coated pit formation. A mechanism was determined for direct iron transport to fetal liver, a major site of hematopoiesis, and to the allantoic sac for storage.