The daily rhythm of body temperature was recorded in control rats fed ad libitum and subsequently fed during daytime 50% of ad libitum food intake. Aside from the expression of a feeding-associated component, body temperature rhythm was phase advanced (7 h) by a timed caloric restriction; the new plateau of the acrophase of the nocturnal peak was close to the light-dark transition. A lesion of serotonergic (5-HTergic) terminals in the suprachiasmatic nuclei (SCN)-the endogenous circadian clock(s)-was performed by microinjection of the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). During the ad libitum-fed state, the acrophase of body temperature rhythm was not modified by the 5,7-DHT treatment. In response to a timed caloric restriction, however, the phase advance of the nocturnal peak of body temperature rhythm was reduced by 2 h in rats with 5,7-DHT lesions as compared to that of sham-operated rats. Magnitude and day-night pattern of wheel-running activity between the two groups of rats also were analyzed. No intergroup difference was found in the amount of wheel-running activity prior to the time of feeding. Moreover, the phase advance of nocturnal component of locomotor activity rhythm observed toward the time of feeding in sham-operated rats was limited by 5,7-DHT treatment. It is concluded that the photic synchronization of body temperature rhythm does not depend on the 5-HTergic projection to SCN under ad libitum conditions. By contrast, the phase-advancing property of a timed caloric restriction on the daily rhythm of body temperature is mediated by a neuronal circuit involving the 5-HTergic projection to SCN. That the phase advance was not fully eliminated by 5,7-DHT treatment suggests that other pathways participate in this mediation.