The compound 2'-thiourea was studied for teratogenic effect in fetal rats. A 0.2% aqueous solution of 2'-thiourea (Merck) was administered ad libitum to pregnant rats from the first to the 14th day of pregnancy. The pregnancy was counted from the day when sperm was found in the vaginal smear. In the treated rat fetuses hypoplasia of the brain and spinal cord, internal hydrocephalus, hypoplasia of the spinal ganglia, kypholordosis, micromelia, micrognathia, cleft palate, retarded tooth development, exophthalmus, coloboma, and cataract were observed. In addition, generalized haemorrhages were found all over the body. The changes were similar to those produced by ethylene thiourea treatment in rat fetuses.