Abstract: It is generillly believed that the effectivenes of salicylates , such as aspirin(ASP) ,is restricted to the lowering of the body temperature(BT ) previously elevated by pyrogensand that salicylates have no effect on normal BT,We present here evidences which demonstrate thatASP does lower the normaI BT of mice kept in a 24℃ environment.Male ICR mice, housed under alight-dark cycle(LD 12 :12h)at a room temperature of 24±1℃ and a humidity of 60±10%withfood and water ad libitum,received intraperitoneal injections of ASP 25,100 and 200 mg· kg^-1.Theanimals showed a significant decline in their rectal temperature and the BT returned to the values thatprevailed before drug administration within 4 h.There was an orderly, progressive dose- dependentdecrease in BT,A significant circadian rhythm was demonstrated for ASP-induced hypothermal effectin mice. Although a significant circadian、rhythm was also demonstrated for the plasma salicylateconcen trations at l h after dosing ,there seems to be no positive relationship between plasma drugconcentrations and the drug response;The results suggest that ASP does affect normal BT regulation inmice and that the circadian rhythm change in ASP-induced hypothermal effect may be mainly due tothe rhythms,in the sensitivity of mice to the drug.