ObjectiveTo investigate the feasibility of in vitro release models of injectable implants. MethodsAn RP-HPLC method was established for determination of norfloxacin (NF). The in vitro release profiles of injectable implants in four release models, including direct injection model, one-way cylinder, dialysis tube and home-made cylinder, were studied and compared with the in vivo release profiles. ResultsThe in vivo release of NF from injectable implants was much faster than the in vitro release of NF. The in vivo release was close to the zero-order release kinetics in the sustained release phase (1-20 d, r=0.993 5), while the in vitro release could be divided into slow phase and fast phase with 15 d as the turning point. Of the four in vitro release models, home-made cylinder showed the closest initial release to the initial release of NF in vivo. ConclusionAll the above four release models have limitations; they can not well reflect the in vivo release profiles of injectable implants; and further studies are needed for the in vitro release model of injectable implants.