The perinatal environment plays a crucial role in programming many aspects of adult physiology. Myriad stressors during pregnancy, from maternal immune challenge to nutritional deficiency, can alter longterm body weight set points of the offspring. In light of the increasing concern over body weight issues, such as obesity and anorexia, in modern societies and accumulating evidence that developmental stressors have long-lasting effects on other aspects of physiology (e.g., fever, pain), we explored the role of immune system activation during neonatal development and its impact on body weight regulation in adulthood. Here we present a thorough evaluation of the effects of immune system activation (LPS, 100 g/kg ip) at postnatal days 3, 7, or 14 on long-term body weight, adiposity, and body weight regulation after a further LPS injection (50 g/kg ip) or fasting and basal and LPS-induced circulating levels of the appetite-regulating proinflammatory cytokine leptin. We show that neonatal exposure to LPS at various times during the neonatal period has no long-term effects on growth, body weight, or adiposity. We also observed no effects on body weight regulation in response to a short fasting period or a further exposure to LPS. Despite reductions in circulating leptin levels in response to LPS during the neonatal period, no long-term effects on leptin were seen. These results convincingly demonstrate that adult body weight and weight regulation are, unlike many other aspects of adult physiology, resistant to programming by a febrile-dose neonatal immune challenge.gender; leptin; lipopolysaccharide; obesity THE PERINATAL ENVIRONMENT is known to play a crucial role in programming body weight. In humans, low birth weight and malnutrition during gestation have been linked with abdominal obesity in adulthood (31,45,46). Similarly, a variety of behavioral, physiological (including dietary manipulations), and immune stressors given to pregnant experimental animals have been reported to alter the body weight of adult offspring (4,15,27,36,40,63). It appears, however, that the timing of perinatal stress is critical in programming many aspects of adult physiology (35) and relatively little is known regarding the influence of postnatal stress on adult body weight regulation.The long-term physiological consequences of a neonatal immune challenge with LPS have been extensively described by our laboratory and others. For example, we have shown attenuated febrile and associated host defense responses in adulthood to an immune challenge of the same type as that experienced at postnatal day (P) 14 (9,17,18). Changes have also been demonstrated in memory processing (7), hypothalamic-pituitary-adrenal axis activation (51, 52), and susceptibility to pathological insults in adulthood (10, 55, 56). However, the long-term effects of a neonatal immune challenge on body weight regulation are less clear, and given the concern over the current obesity epidemic in modern society [e.g., (29)], demand further investigation. For instance, Walker ...