At an ambient temperature (Ta) of 30.degree. C, injection of 0.2 .mu.g Escherichia coli endotoxin into the lateral cerebral ventricle (icv) of 3-day-old or younger guinea pigs was followed by a biphasic febrile rise in body temperature (Tc) and oxygen consumption (VO2), interrupted by a transient fall. At Ta 20.degree. C the change in Tc and VO2 was still biphasic, the 1st rise was similar as that seen at Ta 30.degree. C, but the subsequent fall was more pronounced. Tc became lower than before endotoxin and remained below the pre-injection level during and after the 2nd rise. Injection of 10 ng PG[prostaglandin]E1 icv caused sustained monophasic hyperthermia at both Ta-s. Icv injection of 0.9% NaCl did not affect Tc and VO2 at either Ta. Prostaglandins might contribute to, but cannot account for, the whole febrile response to endotoxin.