Biphasic endotoxin fever in the newborn rabbit.

Rabbits aged 6-10 days responded to 20 or 50 microgram/kg i. p. or i. v. E. coli endotoxin with characteristic biphasic fever, whereas 0-3-day-old rabbits reacted with monophasic fever which coincided with the first-phase rise of the older group. In the rabbit, the mechanisms responsible for the first-phase and the second-phase febrile temperature rises appear separately in ontogenesis in the first week of extrauterine life.