AIM: To evaluate the pharmacokinetics of cyclosporine A(CyA)-loaded chitosan(CS) nanoparticles in rabbits after intravitreal injection. METHODS: Experimental study on pharmacokinetics of sustained release drug in vitreous. Twenty rabbits were intravitreally injected CyA-loaded CS nanoparticles. Liquid vitreous was aspirated at the time of 1 day, 2,3,5,7,9,11,14,21,28 days postoperatively for determination of CyA concentration by high-performance liquid chromatography. RESULTS: 81％ of CyA was released in vitro from CyA-loaded CS nanoparticles after 14 days. The level of CyA ranged from 448.5ng/mL to 1237.7ng/mL during 28 days in vitreous cavities. CONCLUSION: Intravitreal injection of CyA-loaded CS nanoparticles leads to a sustained release of CyA with a high bioavailability. CS nanoparticles is a potentially promising delivery system for the posterior segment diseases.