Abstract:The existence of circadian variations of the toxicity and effectiveness of Guizhi decoction were investigated inrodents. The result shows a significant circadian rhythm for the toxicity of the drug , with the higher toxicity in thelight phase than in the dark phase. A temporal change in the effectiveness of the drughas also been demonstratedwith higher analgesic and febrifuge effects in tke dark phase. All these suggest that the time factor in drug admi-nist ration and the principles of chronopharmacology should be included in the consideration of response to thedrug.