Pregnant rats were utilized to study the effect of maternal hyperthermia on fetal development. Eight groups of six to eight rats were exposed to ambient temperatures of 43-44cC at various stages of pregnancy. All rats were killed on day 20 of gestation. Edema, microencephaly and microph-thalmia followed heat treatment on day 4, 6, or 8 and skeletal defects occurred on day 10 of gestation. Apparently heat stress of dams after day 14 of gestation had little or no effect on embryos. Most placentas from day 6-10 treatment groups were significantly heavier than control and exhibited extensive thickening and necrosis of decidua basalis. Our results suggest that the rat is a useful model for investigating maternal hyperthermia as a possible cause of human placentophathies and fetal retardation.