Unrestrained male Sprague-Dawley rats were infused for seven days with a low (2.45 micrograms/hr) or high (9.81 micrograms/hr) concentration of E. coli lipopolysaccharide (LPS). Compared to control (saline-infused) rats, food intake in the LPS-infused rats remained depressed for the entire infusion period. Despite this long-term suppression of food intake, fever was observed only during the daytime hours for the first two days of infusion. No significant increase in nighttime body temperature was observed. These data indicate that although tolerance to LPS occurred in rats with regard to its fever-inducing effect, tolerance with respect to its anorexigenic action did not occur.