Microparticles are pharmaceutical dosage forms widely used because they may improve the therapeuticapplication of new and conventional drugs. In this work we make biodegradable microparticles withpoly (lactic-co-glicolic) acid as polymer loaded with naproxen. The best microencapsulation conditionsare experimentally determinate. We have found that the drug:polymer ratio, the isopropanol amount,the aqueous phase volume, the stir speed and the time of stir affect the microparticles characteristics. Inthe carragenin paw edema model, we have found that the microencapsulated naproxen at 5 mg/Kg and10 mg/Kg showed greater effect than free naproxen at 10 mg/Kg.