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In this study, the use of biodegradable polymers for microencapsulation of naltrexone using solvent evaporation technique is investigated. The use of naltrexone microspheres for the preparation of matrix devices is also studied. For this purpose, poly(L-lactide) (PLA) microspheres containing naltrexone prepared by solvent evaporation technique were compressed at temperatures above the Tg of the polymer. The effect of different process parameters, such as drug/polymer ratio and stirring rate during preparation of microspheres, on the morphology, size distribution, and in vitro drug release of microspheres was studied. As expected, stirring rate influenced particle size distribution of microspheres and hence drug release profiles. By increasing the stirring speed from 400 to 1200 rpm, the mean diameter of microspheres decreased from 251 μm to 104 μm. The drug release rate from smaller microspheres was faster than from larger microspheres. However, drug release from microspheres with low drug content (20% wt/wt) was not affected by the particle size of microspheres. Increasing the drug content of microspheres from 20% to 50% wt/wt led to significantly faster drug release from microspheres. It was also shown that drug release from matrix devices prepared by compression of naltrexone microspheres is much slower than that of microspheres. No burst release was observed with matrix devices. Applying higher compression force, when compressing microspheres to produce tablets, resulted in lower drug release from matrix devices. The results suggest that by regulating different variables, desired release profiles of naltrexone can be achieved using a PLA microparticulate system or matrix devices.