With both scanning and transmission electron microscopy, large ectoplasmic projections are found intruding into the lumen from the epithelial cells of the rat uterus. These projections, which are abundant both prior to implantation and during delayed implantation, communicate with the underlying cytoplasm only by a small pedicel as though in the process of pinching off. However, introduction of tracer material into the uterine lumen demonstrates the pinocytotic nature of these projections. Within three minutes after introduction of tracer the projections, termed pinopods, contain numerous vacuoles filled with tracer. Within ten minutes large vacuoles containing tracer are present in the apical cytoplasm subjacent to the individual pinopods. The varied images observed in the experimental and control materials suggest that there is a continual turnover of pinopods. Initially a simple ectoplasmic projection, the pinopod apparently develops rapidly into a mass of ectoplasm 2–3 μ in diameter with multiple folds and pockets at its surface and numerous internal vacuoles. Following a period of active endocytosis of fluid, the pinopod becomes more spherical and, together with contained material, is withdrawn into the apical cytoplasm. It is suggested that pinocytosis might play a role in producing apposition of the blastocyst to the luminal epithelium, in passing information from the blastocyst to the stroma, and in diminishing the molecular contents of the uterine lumen during specific times in the reproductive process.