AbstractA number of viruses are capable of producing intrauterine infection and damage. Both rubella and cytomegalovirus may cause malformations. For rubella the most frequent defects are those involving the heart, eyes, brain, and ears. With cytomegalovirus, children may be microcephalic, small, and mentally retarded. The mechanism of teratogenesis for rubella is apparently related to the direct infection of the virus in certain tissues of the developing fetus. Indirect effects may also be caused by vascular occlusion due to damage of blood vessels. Studies of rubella in experimental animals have not been particularly successful. Isolated reports have suggested that congenital damage can be produced in monkeys and rats. Further studies, however, are necessary.A number of “model systems” are available for the study of viruses in experimental animals. It is now possible to utilize basic guidelines in consideration of experimental teratogenesis in these systems. These guidelines include: 1. Infection should be produced in the adult and fetus or newborn animal. 2. In most cases the virus used should be low passage, well adapted to the animal, and preferably passaged in the living animal. 3. Several different intervals during gestation should be studied, including the inoculation of newborn young. Several dosages and routes should be tried. 4. Indirect effects on the mother or infant, environmental factors, mating patterns, and genetic background variables should be considered. 5. The effects should be prevented by specific neutralization of the virus.