AbstractNasolabial morphogenesis in Long‐Evans rats was studied both antenatally and postnatally. Microscopically, the facial processes corresponded to areas where mesenchyme had condensed to form growth centers. Teratogens such as methyl salicylate, trypan blue, and 9‐methyl PGA when administered to pregnant rats principally caused underdevelopment of the maxillary processes and failure of mergence of the nasomedial processes in the young so that a variety of clefts resulted. In addition, some experimental embryos showed shallow grooving of the maxillary process, which corresponded with the location of maxillary cleft, and abnormality occasionally seen at a later stage in man and other anim