California ground squirrels (Spermophilus beecheyi) in the natural environment maintain a higher mean body temperature (T_b) compared with animals maintained in the laboratory. Our goal was to determine whether that increase was the result of an elevation in T_b set point analogous to a fever induced by acute stress. Body temperatures were measured by telemetry in a paired experiment on 10 animals maintained in the laboratory, with each animal receiving an injection of sterile pyrogen-free saline and 50 μg/kg endotoxin of E. coli. Body temperatures also were measured by telemetry on 8 pairs of free-living animals, with 1 member receiving an injection of sterile saline and the other animal receiving an injection of 50 μg/kg endotoxin of E. coli. Animals maintained in the laboratory exhibited a fever following injection of endotoxin, with significant elevation in T_b from 6 to 9 h after injection. Free-living animals that were injected with endotoxin did not exhibit an increase in T_b compared with saline-injected controls over the same time. Absence of a febrile response in free-living California ground squirrels supported the hypothesis that this large squirrel had a pre-existing elevation in thermoregulatory set point.