Ciliary neurotrophic factor (CNTF), an inducer of neurogenesis in the hypothalamus, has been known to cause a permanent fall of body mass in mice made obese by a fat-rich diet. In the present study, energetics of obese mice was followed during and after a 7-days long intracerebroventricular infusion of CNTF (720 ng/day) using an ALZET minipump. The animals were previously implanted with MINIMITTER biotelemetry transmitter allowing monitoring of abdominal core temperature (Tc) and locomotor activity (Act). The fat-rich diet induced a rise in body mass by about 40% over a period of 2 months and led to a prompt decrease of circadian Tc excursions by about 50% and an increase of 24-h Tc averages without a change in Act. Infusion of CNTF resulted in an expected reduction of body mass of obese mice beyond the period of infusion. This response was accompanied by a rise in daily averages of Tc together with a decrease in daily Tc excursions and a fall in Act. The observed fall of body mass, rise of Tc and probably a decrease of food intake belong to the components of sickness behaviour.