Objective： To investigate the in vivo absorption kinetics of caffeine at different intestine segments in rats. Methods： The rat intestine was cannulated for in situ recirculation. The absorption kinetics of caffeine,including the absorption segments of intestine,drug concentration and p H value,were investigated in all the rats under light anesthesia. Ultraviolet light spectrometry and high-performance liquid chromatography were used to determine the concentrations of phenol red and caffeine, respectively. Results： The absorption rate constants （Ka） of caffeine in the small intestine at the concentrations of 2.5,5,10 and 25 mg· L^-1 were0.252,0.247,0.304 and 0.232 h^-1,respectively. The Ka values at pH of 7.8,6.8,and 5.4 were 0.267, 0. 274 and 0. 247 h^-1 ,respectively. The Ka values in the duodenum,jejunum,ileum,and colon were 0. 112,0. 099,0. 095 and 0. 069 5 h^-1 ,respectively. Conclusion, Concentration and pH value have no obvious effect on the absorption kinetics of caffeine. Caffeine can be better absorbed in the duodenum,jejunum and ileum than in the colon. The absorption of caffeine in the intestine is a first-order process through passive diffusion mechanism.[