An interpretation of the drug release from monolithic water-swellable and soluble polymer tablets is presented. A convenient parameter, alpha, which compares the drug-diffusive conductance in the gel layer with the swelling and dissolving characteristics of the unpenetrated polymer was used to describe the release behaviour of beta-hydroxyethyl-theophylline (etofylline) from compression-moulded tablets of hydrophilic pure semicrystalline poly(ethylene oxides) of mol wt 600,000 and 4,000,000 and of two blends of the two molecular weights of poly(ethylene oxides). The water swelling and dissolution characteristics of two polymers and two blends were analysed, monitoring the thickness increase of the surface-dissolving layer and the rates of water swelling and penetration in the tablets. The drug diffusivities in the water-penetrated polymer gels were measured by carrying out permeation tests. Finally, drug release tests were performed to investigate the release kinetics of the different systems in an aqueous environment at 37 degrees C. The drug release from the high molecular weight poly(ethylene oxide) is principally related to the material swelling rather than polymer dissolution, leading to a progressive decrease of the drug's diffusive conductance in the growing swollen layer, and hence to a non-constant release induced by the prevailing diffusive control. Conversely, drug release from the low molecular weight poly(ethylene oxide) is strictly related to the polymer dissolution mechanism. The achievement of stationary conditions, in which the rate of swelling equals the rate of dissolution, ensures a constant release rate, even in the case of very low drug-diffusive conductance in the external gel layer. Intermediate behaviours were detected in the case of the two blends.