The aim of this study is to reveal statistically how various geometrical shapes such as triangle, cylinder, half-sphere affect the release rate of the active substance called theophylline in erodible hydrogel matrix tablets. We have tried to indicate these changes in the release rate of theophylline by supporting our aim with the mathematical equations developed by Hopfenberg and Katzhendler et al. The model developed by Hopfenberg assumes that drug release occurs from the primary surface area of the device but Katzhendler et aI. (I. Katzhendler, A. Hoffman, A. Goldberger, M. Friedman, Modelling of drug release from erodible tablets, J. Pharm. Sci. 86 (1997) 110-115), described a situation where the erosion rates of the tablet are different in the radial and axial directions. Hydrogel matrix tablets were prepared with hydroxypropylmethylcellulose (HPMC E-50) possessing different geometrical shapes as triangular, cylindrical and half-spherical using experimental design. When the dissolution results have been evaluated, it has been observed theophylline release from different geometrical erodible tablets fitted with that of the Katzhendler et al., equation. This equation which was suggested for cylindrical tablets was also used to interpret half-spherical and triangular tablets. According to the above stated equation, n has been determined as 4 for triangular tablets and 1.5 for half-spherical tablets and we have also suggested that, these n values could be used in the kinetic programs. (C) 2000 Elsevier Science B.V. All rights reserved.