Sustained release microspheres of chlorpromazine were prepared from Eudragit(R) RS 100 by an emulsion-solvent evaporation method using a six-baffled vessel. The morphology of microspheres was characterized by scanning electron microscopy (SEM). In the presence of aluminium tristearate (5%), microspheres were spherical in shape and uniform. The release of chlorpromazine from microspheres was pH-independent. With increasing amount of aluminium tristearate and increasing ratio of Eudragit(R) RS 100/drug from 1:1 to 9:1, the particle size of chlorpromazine microspheres was reduced and the release rate decreased. The microspheres prepared with the polymer/drug ratio of 9:1 produced a 12-h sustained release pattern.