The growth and fusion of facial processes were studied in White Leghorn chicken embryos treated with hydrocortisone. This treatment results in complete bilateral cleft beak in 80-100% of cases. The frontonasal complex of the experimental embryos exhibited marked hypoplasia prior to cleft manifestation. During the critical period for cleft beak formation, the maximum mitotic activity was concentrated in the medial nasal processes, which thus became vulnerable to the mitosis inhibiting effect of corticoids. Primary hypoplasia of the facial processes was documented as a causative factor of corticoid-induced cleft beak which is analogous to mammalian cleft lip/palate.