The effect on development of early mouse embryos of making a hair-line slit in the zona pellucida of approximately one-third its diameter was investigated. The rate of development to mid-gestation of operated zygotes and 2-cell embryos transferred directly to the oviduct was significantly lower than that of sham-operated or unoperated controls. However, the operation had no discernible effect on the development of 2-cell embryos that were cultured for 2 days prior to transfer to the uterus, or on embryos composed of 8 or more cells transferred directly to the oviduct. Zona slit zygotes and 2-cell embryos exhibited a significantly higher rate of anomalous development to the morula or blastocyst stage than controls following short-term transfer to the adult or immature oviduct. Such anomalies could not be attributed to damage of the embryos by leucocytes or bacteria entering through the wound in the zona. Rather, the typically non-spherical shape of slit zonae, together with the fact that some were empty on recovery, was consistent with operated embryos having been damaged by compression during passage through the oviduct. This suggests that, providing it is intact, the zona pellucida protects the early embryo from contraction of the oviductal musculature which is sufficient to lyse, arrest or extrude blastomeres prior to the formation of intercellular junctions. Hence, in experimental manipulations entailing damage to the zonae of early embryos, there may be a case for allowing them to form morulae in vitro prior to transfer, rather than returning them directly to the oviduct.