Pigeons show a profound circadian rhythmicity of core temperature (Tcore). The Tcore is about 43 degrees C during the daytime and about 40 degrees C at night. Intravenous injections of bacterial endotoxin (LPS: lipopolysaccharide) during the day and night in pigeons evoked fevers of different magnitude. When LPS was injected during the day, Tcore first decreased from 41.8+/-0.2 degrees C to 41.4+/-0.3 degrees C, then started to increase after about 1.5 hrs, reaching 42.9+/-0.2 degrees C about 5 hrs after the injection. When LPS was injected at night, Tcore immediately increased from 40.3+/-0. 2 degrees C to 41.8+/-0.3 degrees C within 5 hrs after the injection. Fever continued for about 13 hrs when endotoxin was injected during the day, but only for about 9 hrs when it was injected at night. Compared with the diurnal pattern of afebrile core temperatures, fever was superimposed on the circadian rhythmicity. This suggests that the control of circadian rhythm of core temperature is not disturbed during fever.