Guanabenz (WY-8678) was given to pregnant rabbits to examine its effects on the dams and their fetuses. The drug was administered orally to rabbits at does of 10, 8, 5 and 1 mg/kg/day from the 6th to 18th days of gestation. The vehicle used was 5%-Arabic Gum at dose level of 10 ml/kg, and a control experiment where only the vehicle was administered was also conducted. The results obtained were as follows. 1. Concerning the effects on the dams, the symptoms of decrease in spontaneous movement and sedation were observed at 10 mg/kg. However, mean body weight, food consumption, and organ weight were not influenced by Guanabenz. 2. In the observation of the fetuses, no difference was observed among groups in the number of corpora lutea, the number of implantations, implantation rate, the number of resorption, the number of dead fetuses, the number of live fetuses, sex ratio, mean body weight of the fetuses, mean body length of the fetuses, and mean placenta weight of the fetuses. 3. No difference was observed among groups regarding external malformations, visceral malformations, skeletal malformations, skeletal variations and degree of ossification in the fetuses. From these results, it is concluded that Guanabenz has no teratogenic potential in pregnant rabbits and on effect no fetuses at doses of 8 mg/kg or lower.