Maternal-to-infant transmission of simian immunodeficiency virus (SIV) has been demonstrated in the rhesus macaque following experimental infection of pregnant rhesus monkeys, either parenterally or by inoculation of virus into the amniotic fluid. Virus infection occurred in 3 of 12 (25%) rhesus infants born to mothers with SIV infection induced by parenteral inoculation of virus during gestation. However, these infants did not become seropositive or virus positive until they were 9-15 months old, suggesting that virus infection most likely occurred as the result of breast feeding. Infection has also been demonstrated in one rhesus infant following virus inoculation into the amniotic fluid during late gestation. These observations support the increasing evidence that intrapartum or postpartum infection may be important mechanisms for the maternal-infant transmission of HIV. The SIV-infected macaque should prove to be a useful model to evaluate the timing and mechanisms of lentivirus infection in infants, to determine maternal factors associated with transmission to the fetus or infant, and to evaluate therapeutic regimens for the prevention or treatment of pediatric AIDS.