To prepare sustained phenylpropanolamine*HCl (PPA*HCl)-resin complexes and to study the factors influencing the in vitro dissolution of it. METHOD: The sustainedPPA*HCl-resin complexes were prepared by coating the complexes with different kinds of ethylcellulose (EC) as coating embranes. The factors influencing the release rate of PPA*HCl from the complexes, such as resin particle size, the viscosity and thickness of coating membrane, the dispersion medium of EC (water or alcohol) as well as thepH and ion strength of dissolution medium were studied in detail. RESULTS: Thefactors above all influenced the dissolution of the complexes to ifferent extent. When prepared by EC 200 Pa*s (150 g*kg-1), the dissolution of the coated complexes in 0.1 mol*L-1 HCl accorded with the requirements of sustained PPA*HCl capsulesof USP ⅩⅩⅢ. ONCLUSION: The sustained PPA*HCl can be prepared by coating the drug-resin complexes with EC.