Penetration of the zona pellucida of the rat, mouse, and individual representatives of other species, as well as the extracellular coats of the rabbit, by exogenous protein was studied using horseradish peroxidase and ferritin as tracers. In addition, possible variation in penetration in relationship to time of exposure and preimplantation stage of development was studied in the rat and rabbit. In all species studied, both peroxidase (mol wt 40,000) and ferritin (mol wt at least 500,000) had penetrated when examined by electron microscopy whether the exposure was 10 mm or longer and regardless of stage of development. It is concluded that the zona pellucida does not exclude molecules on the basis of size within the normal range of individual protein molecules.