It has been demonstrated that the ferret (Mustela putorius x Mustela putorius furo) responds to intramuscular injection of Salmonella typhi lipopolysaccharide (30 ng/kg-100 micrograms/kg) by biphasic change in the body temperature (Tb): the initial decrease in the latter is followed by hyperthermia. Maximum rise in Tb (1.6 +/- 0.1 degrees C) was observed after the injection of lipopolysaccharide in the highest dose. Rabbit leucocytic pyrogen/interleukin-1 (1 ml from 3.5 x 10(7) peritoneal phagocytes, 3 ml/kg) induces a pronounced (1.1 +/- 0.3 degrees C) decrease in Tb. Mechanisms of hypothermic effects of pyrogens are discussed. The described pattern (hypothermia-hyperthermia) of Tb response to lipopolysaccharide in the ferret presumably reflects the central thermoregulatory process which is the same for different changes in Tb during fever.