The mechanism of papaverine hydrochloride release from ethyl cellulose-walled microcapsules in both simulated gastric and intestinal fluid is discussed. The microcapsules were prepared by coacervation using different core: wall ratios. The rupture of the thin-walled microcapsules after release in simulated gastric fluid was shown and attributed to the internal osmotic pressure, supporting a mechanism for drug dissolution. The internal osmotic pressure produced only a few small holes in the thin-walled microcapsules after release in simulated intestinal fluid. No rupture of the thick-walled microcapsules after release in either medium was shown. Therefore these release data fitted diffusion-type kinetics. It is suggested that the internal osmotic pressure developed after penetration of the medium is affected by the ratio between the core dissolution rate and the drug diffusion rate through the wall.