It is estimated that up to 40% of mammalian embryos die prior to implantation. The causes of early embryonic death are poorly understood, despite widespread recognition of this phenomenon and research efforts to determine the reasons for this significant wastage of embryos. The determination of factors that cause embryonic losses is hampered by the lack of reliable methods to evaluate the viability of the embryo prior to implantation. The major objective of this dissertation was to develop an assay, based on the exclusion of eosin B, for the assessment of the viability of preimplantation embryos. The rat embryo was used as the experimental model for these studies;Heating of rat embryos to 55°C blocked in vitro development and killed the embryos. Dead embryos were completely stained when exposed to a 120 [mu]M, 600 [mu]M, or 1200 [mu]M solution of eosin B. The interval from dye exposure to complete staining of dead embryos ranged from 1 to 14 minutes and was influenced by the concentration of dye and the stage of embryonic development. A single exposure of 4- to 8-cell embryos to 120 [mu]M or 600 [mu]M eosin B prior to culture or multiple exposures to 120 [mu]M eosin B during culture did not influence the in vitro development of embryos. However, continuous culture in medium which contained 120 [mu]M eosin B reduced the number of embryos that cleaved and continuous exposure to 600 [mu]M eosin B was toxic to the embryo. A single exposure of morulae and blastocysts to the 600 [mu]M concentration of eosin B allowed for the discrimination of viable and nonviable embryos, but did not affect the viability of embryos after transfer to synchronized recipients. Live offspring were born when embryos, which were unstained when exposed to eosin B, were transferred into the uterine horns of pseudopregnant or naturally mated recipients. Thus, the embryos which were shown to be alive by not staining with eosin B competed successfully for implantation sites with the "self" embryos of the naturally mated rats and offsprings were born;The results of these studies conclusively indicate that the eosin B dye-exclusion assay is a reliable method to evaluate the viability of rat embryos. The staining response of embryos to eosin B can be used alone, or in combination with the development of the embryo, for the discrimination of embryostatic and embryolethal effectors.