Inoculation of random-bred mice with Sindbis virus at various periods of pregnancy resulted in the development of marked changes and loss of 24.7% of fetuses when mice were infected at the stage of placentation. Retardation in the development and death of progeny in females infected at later stages of pregnancy were observed. Antibody production was shown to be enhanced in pregnant mice as compared with non-pregnant. Possible mechanisms of increased susceptibility to alphaviruses during pregnancy and in early stages of ontogenesis are discussed. A potential danger of development of embryo- and fetopathies in humans is considered on the basis of the above studies and the data on equivalent stages of prenatal development of mice and human beings.