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Prolonged food deprivation is known to cause a fall in the core body temperature of homeotherms. In various species of small birds and mammals (body mass up to 2-3 kg), it has been shown that starvation-induced hypothermia is modulated by the circadian system, in the sense that hypothermia is observed primarily during the inactive phase of the daily activity cycle (i.e., during the night for diurnal animals and during the day for nocturnal animals), whereas relatively normal temperatures are recorded during the active phase. To investigate whether this modulation occurs also in larger animals, we investigated the effects of 4 d food deprivation on the body temperature rhythm of goats and sheep (body mass 30-40 kg). In goats, the body temperature rhythm was found to have a mean level of 39.0 degrees C with a mean daily range of excursion of 0.42 degrees C. The daily oscillation in body temperature persisted during the first day of fasting, but the rhythm was drastically damped, if not eliminated, over the next 3 d as body temperature descended from the baseline level of 39.0 to 38.2 degrees C. In sheep, the rhythm was found to have a mean level of 39.3 degrees C with a mean daily range of excursion of 0.34 degrees C. The daily oscillation in body temperature persisted through the 4 d of food deprivation, even though the mean level of body temperature gradually fell. Temperature fell more during the third and fourth nights than during the third and fourth days. Thus, circadian modulation of starvation-induced hypothermia was observed in sheep but not in goats.