Hypothalamic injection in the rats of bacterial endotoxin or endogenous pyrogen from rabbit's leucocytes induced fever. During the fever thus induced, the rats were exposed to radiant heat. Their behavioral thermoregulation, determined by the bar-pressing rate to escape from the radiant heat, was significantly reduced compared to that in the rats untreated with the pyrogens and exposed to the radiant heat. Thus, the rats with the induced fever prefered high levels of tail-skin and ambient temperatures, signs of the development of behavioral fever.