Biodegradable membranes containing progesterone as a drug were prepared from ternary, poly(d,l-lactide-co-glycolide)/progesterone/dimethylformamide, solutions. The homogeneous solutions, after cast on glass plates, were solidified to result in a solid membrane structure by three different solvent-removal processes: solvent evaporation under vacuum, solvent extraction via immersion into the nonsolvent bath, or vapor exposure at high humidity condition. Impregnation characteristics of progesterone in the prepared membranes varied significantly, depending on the removal processes used. When a cast solution was solidified by exposure at the environment of 70% relative humidity, progesterone was separated from a membrane structure with the morphology of flake-like shapes, and thermal analysis of the prepared membrane showed the clear, endothermic peak of the drug. Vitrification of a cast solution by solvent evaporation under vacuum induces both the uniform drug dispersion in the polymer matrix, with the drug forming spherical structures, and the strong interaction between the drug and the matrix, as identified by a broadened melting endotherm of the drug. When coagulated at thermodynamic nonequilibrium conditions through rapid exchange between dimethyformamide and water, the cast solution film results in a membrane structure consisting of the drug distributed nonuniformly in the polymer matrix. (C) 2000 John Wiley & Sons, Inc.