Although peripherally released interleukin (IL)-10 has a critical regulatory role in limiting fever in mild-to-moderate forms of inflammation, its role in regulating the more complex thermoregulatory manifestations of hypothermia and fever noted during severe inflammation is less clear. Using cytokine antagonism, we therefore investigated the involvement of peripherally released IL-10 in mediating hypothermia, fever and inflammation induced by intraperitoneal (IP) administration of a large dose of lipopolysaccharide (LPS). Male Wistar rats (200–250 g) were anaesthetized and implanted intra-abdominally with temperature-sensitive radiotelemeters. Rats were randomly assigned to receive IL-10 antiserum (IL-10AS) or normal sheep serum IP, 4 h before receiving an IP injection of LPS (10 mg/kg) or phosphate-buffered saline (PBS). Inflammatory responses were measured in plasma and tissue samples (spleen, liver and brain) at 90 min and 6 h after the IP injection of LPS or PBS. Administration of LPS induced an initial period of hypothermia (~90 min) after which fever developed. Pre-treating rats with IL-10AS abolished the LPS-induced increase in plasma IL-10 levels, attenuated the hypothermia and increased the amplitude of the fever. Moreover, IL-10AS pre-treatment augmented the LPS-induced increase in plasma levels of tumor necrosis factor-alpha (90 min and 6 h), IL-1β (90 min), prostaglandin E₂ (90 min) and IL-6 (6 h), in the periphery, but not the hypothalamus, over the duration of hypothermia and fever. Via its action on the synthesis of inflammatory mediators in the spleen and liver, endogenous IL-10 plays a crucial regulatory role in mediating hypothermia and fever during severe aspectic (LPS-induced) systemic inflammation.