BACKGROUND: 5-fluorouracil (5-Fu) is universally used as an antineoplastic agent in gastrointestinal cancer, but the side effect of it confined further clinical application. OBJECTIVE: To determinate mice plasma concentration curves for 5-Fu and its chitosan (CS)-polyaspartic acid (Pasp) nanoparticles, and to investigate their primary pharrnacokinetics. DESIGN, TIME AND SETTING: Randomization control animal trials were performed in the Department of Gastroenterology, Zhongshan Hospital of Fudan University between October 2006 and June 2007. MATERIALS: Totally 180 female Kunming mice were obtained from the Department of Laboratory Animal, Fudan University. 5-Fu (purity 99%) was purchased from Shanghai Xudong Haipu Pharmaceutical Co, Ltd (Shanghai, China). Two kinds of CS-Pasp-5-Fu particles were offered by Department of Macromolecular Science, Key Laboratory of Molecular Engineering of Polymers of Educational Ministry, Fudan University (Shanghai, China). METHODS: Kunming mice were randomly divided into three groups and each group was administrated with 5-Fu or either type of its CS-Pasp-5-Fu nanoparticlos. The plasma concentrations of 5-Fu were evaluated by high performance liquid chromatography after 15 minutes, 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 12 hours, 16 hours, 24 hours and 48 hours of the administration. The pharmacokinetic parameters were computed utilizing 3P97. MAIN OUTCOME MEASURES: Relative recovery, absolute recovery and the stability of samples. RESULTS: The peak concentration of 5-Fu group occurred within 15 minutes and then decreased rapidly. The No.1 nanoparticles group's peak concentration occurred 6 hours after the administration and the effective concentration time lasted for about 14 hours. No.2 nanoparticles group's concentration curve was double-apex, the apexes occurred around the 2 hours and 16 hours, the concentration decreased at the 24 hours after the administration. Both of the two kinds of the nanoparticles groups' peak concentration of 5-Fu in plasma are lower than the 5-Fu group, The half-life times were prolonged and the areas under curve were higher. CONCLUSION: Compared to 5-Fu, the CS-Pasp-5-Fu nanoparticles are controlled released.