Metabolic alterations related to resting energy expenditure (REE) and fat metabolism have been noted during sepsis, and often depend on the causative agent and the stage and severity of the illness. We studied the effect of IL-1, the protein mediator of the 'acute phase' response during infection, on REE, respiratory quotient (RQ), and fat metabolism in male rats (210 g), who were infused over an 8-h period with (1-14C)-palmitate (PALM), (2-3H)-glycerol (GLY) and either saline or interleukin-1 (IL-1). At 7-8 h post infusion, the IL-1 group showed a significant increase in REE but no change in RQ. The IL-1 group also exhibited a significant decrease in serum free fatty acid (FFA) and an increase in FFA clearance. Free fatty acid flux, %PALM oxidation, serum (GLY), glycerol clearance, and glycerol flux (a measure of lipolytic rate) were not significantly different between the two groups. We conclude that IL-1 can mimic the increase in REE seen during infection; the increase in REE is not due to a selective increase in fat oxidation only, although the unchanged RQ and increased REE suggest that there is a proportional increase in net FFA oxidation.