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The purpose of this study was to prepare microspheres containing Huperzine A, which is used for patients suffering from Alzheimer's disease because of its potent anticholineestase activity, and to clarify in vitro and in vivo release characteristics of them.The preparation and in vitro and in vivo release studies of Huperzine A loaded microspheres were described. By spray drying method, Huperzine A was encapsulated successfully in the microspheres which were spherical with a non-porous and smooth surface. In vitro studies showed that the release of Huperzine A from microspheres was depended on the properties of polymers and the release medium. Counter-ionic interaction between the primary amine group of Huperzine A and the carboxylic terminal group of PLG polymers improves the encapsulation of Huperzine A, reducing the initial burst and extending the sustained release. High molecular weight of PLG polymer leads to a negative influence on sustained release of Huperzine A due to less carboxylic terminal groups. Acidic medium also reduces the initial burst and sustained the release due to decreased swelling of the polymeric matrix. In vivo experiment showed, after intramuscular injection, that the plasma concentration of Huperzine A reached the max. at 2 h, then fell rapidly to a stable and near constant level of 0.5 to 2.5 ng/ml within 2 weeks, until the drug was exhausted from the microspheres. It indicates the potential of a 2-week sustained release system of Huperzine A.