Animal studies of endotoxemia during pregnancy have suggested that the placenta is the primary site of action. This study was designed to evaluate the effects of endotoxin at previously established resorptive and teratogenic dose levels during the development of the placenta in the hamster. At the higher dose levels, total necrosis of the placenta was observed within 24 hr after treatment. Formation of fibrin thrombi was only apparent well after the initiation of necrotic events. At the teratogenic dose, a marked decrease in the formation of the placental labyrinth was evident, but invading fetal capillaries and fetal blood cells appeared normal. However, scattered necrotic foci of trophoblastic cells were observed.