AbstractThe development of cleft palate after x‐irradiation was studied in mice of the C57BL strain. Progress in palate development was evaluated with reference to chronological age, morphological rating and weight of the embryos. Movement of the palatine shelves from the sagittal to the horizontal plane was found to have been retarded by x‐irradition. Measurements of head and palate did not show any consistent disproportionality of palate growth in the x‐irradiated embryos except that which resulted from retardation of shelf movement.X‐irradiation affected A/Jax strain litters more severely than C57BL strain litters according to cleft palate frequency and average palate stage at eighteen and one‐third days postc