The acute LD50 in virginal NMRI-Mice was found to be 2528 mg/kg for monolinuron and 1791 mg/kg for buturon. Pregnant female mice of the NMRI strain were administered orally 25--1000 mg monolinuron/kg (I) and 100-400 mg buturon/kg (II) on days 6--15 of gestation and, during defined phases of fetal development (days 10--13 after conception), 500 mg I/kg and 350 mg II/kg. Following administration of 10 doses, an increase of postimplantative losses and clear retardation of development in the upper dose range from 100 mg I/kg and 300 mg II/kg as well as a dose-dependent increase of the rate of cleft palates could be observed. High doses of both substances given from day 6--15 of pregnancy produced minor numbers of wavy and fused ribs as well as hypoplasia of the upper jaw after application of monolinuron and exenteria and exencephaly after buturon. Administration of monolinuron between days 10 and 13 of gestation resulted a minor and that of buturon a clear increase of the number of cleft palates. To evaluate postnatal development 200 and 500 mg I/kg, and 200 and 300 mg II/kg were administered orally on days 6--15 of gestation. In the higher doses, both substances produced an increased mortality among the offspring up to 3 weeks after birth, and a clear increase of the rate of cleft palates.