The teratogenic effect of cyproheptadine chlorhydrate was studied in Wistar rats. Rats were given the substance by gastric intubation on days 6-15 of pregnancy in two experimental series. In the first series rats were treated at doses of 25 and 50 mg/kg/d and in the second one they received 15, 25 and 35 mg/kg/d. Controls received only an equivalent volume of water by the same route. Doses of 25 or more mg/kg/d are highly embryotoxic. At 15 mg/kg/d the effects were relatively minor. The major anomalies caused by the drug were edema and abnormal ossification of the ribs. Some other malformations were also found: craniorrhachischisis, cleft lip, cleft palate, hypoplastic limbs, micrognathy, micromelia, and vascular damage.