Behavioral fever in newborn rabbits.

Rabbit pups, 12 to 72 hours old, did not develop a fever when injected intraperitoneally with a pyrogen and maintained at an ambient temperature of 32 degrees C for 2 hours. When placed in a thermally graded alleyway, animals injected with pyrogen selected gradient positions that represented significantly higher temperatures than controls injected with saline (40.4 degrees in contrast to 36.4 degrees C). Allowing the pups to remain at their selected positions for 5 minutes caused a significant increase in the rectal temperatures of the pyrogen-injected pups but not that of the controls. Thus, newborn rabbits will develop a fever by behavioral means after a single injection of an exogenous pyrogen.