This paper describes the ultrastructural appearance of guinea pig blastocysts that are in the process of shedding the zona pellucida, and it attempts to relate these observations to hypotheses regarding the mechanism of zona shedding. Blastocysts were flushed from the uterus at about the time of zona shedding on Day 6 of pregnancy. Out of a large number of embryos collected and examined with the light microscope, four which were in the process of shedding the zona and three which were not were examined with the electron microscope. Projections from the implantation cone into the zona pellucida were found to contain a system of microfilaments oriented parallel to the direction of penetration of the projection, and variable numbers of small, smooth-walled vesicles. Other organelles were absent from the projections. The zona pellucida around the projections appeared uniformly dense, and there was no indication, either from the appearance of the zona or from the organelles of the cells, that a proteolytic enzyme was secreted to digest the zona pellucida. Most of the surface of the implantation cone bordering on the zona pellucida showed evidence of surface membrane activity; microvilli and coated (pinocytic) vesicles were abundant. It is suggested that pinocytosis of the zona material may be involved in the formation of the hole through which the embryo emerges as the zona is shed.