We tested the effect of perinatal (one week prenatal and one week postnatal) normobaric hypoxia on the immune response of rats in their 9th week of life. We found that perinatally hypoxic rats produced less serum antibodies after sequential immunization with ovalbumin and sheep red blood cells. Also phagocytosis of HEMA microparticles by neutrophil leukocytes from perinatally hypoxic rats was depressed as well as the oxidative burst of their peritoneal macrophages and neutrophils. These results demonstrate that perinatal hypoxia has an important effect on the immune system of the rat.