During infections, plasma Fe and Zn generally fall, while body temperature and plasma Cu rise. However, infected neonates usually do not develop fever during the first week of postnatal life. While fever could not be evoked in neonatal guinea pigs by 2 micrograms/kg of S. enteritidis endotoxin until they were 8 days old, their plasma levels of Fe and Zn were lowered significantly from birth; plasma Cu tended to increase from 2 days postnatally. These results indicate that, contrary to the refractoriness to endotoxin of the fibrific system, the ability to alter trace metal levels exists from birth. Thus, fever and trace metal levels are not necessarily coupled for host defense during infection.