Drug release from spherical matrix systems has been investigated theoretically, with numerical as well as analytical methods. The model used combines the Noyes-Whitney and diffusion equations, and thus takes the effects of a finite dissolution rate into account. The release profile has been determined numerically, by using well-established FORTRAN routines. An approximate analytical formula for the amount of released drug has been derived, which is valid during the early stages of the release process. This analytical short-time approximation was compared to the numerical result, and to drug release models existing in the literature. From this comparison it was concluded that the analytical approximation provided a good description of the major part of the release profile, irrespective of the dissolution rate. Existing literature models, based on instantaneous dissolution, provided a good description of the release only when drug dissolution proceeded very rapidly in comparison with the diffusion process. Consequently, the new analytical short-time approximation complements the formulas existing in the literature, since it provides a superior description of the release of slowly dissolving drugs.