For the effective administration of naloxone, we attempted to investigate the naloxone sustained-release implants. Using the biodegradable polymer, poly[(diethyl glutamate)-co-(ethyl glycinate)phosphazenes](PGGP), the implantable devices containing naloxone hydrochloride(NLX HCl) and naloxone base(NLX) were prepared. The release rates of NLX and NLX HCl were compared. Influences of NLX contents on release rates were examined. For pharmacokinetic studies, NLX and NLX HCl loaded devices were implanted subcutaneously in rabbits and then the plasma concentrations of NLX were determined by HPLC(ECD). NLX-containing devices were implanted with various doses and pharmacokinetic parameters according to dose were calculated. The relative bioavailabilities were evaluated and compared. Incorporation of NLX in the polymer leaded to a slow release. There were no differences of release rates based on drug contents. In pharmacokinetic parameters determined in 216 hours, NLX loaded devices resulted in enhanced bioavailability with the higher AUC (p<0.01) than NLX HCl loaded devices and MRT was significantly (p<0.05) increased. This result demonstrates that NLX is more suitable for sustained release devices than NLX HCl. Therefore it is anticipated that the effective concentrations of naloxone could be maintained for longer periods and bioavailabilities could be improved by naloxone sustained-release implants, with varying drug base/hydrochloride.