An inward release polymer matrix with its aperture covered by a drug-permeable membrane is proposed as a potential zero-order controlled release device. The presence of the membrane has the effects of modifying the rate of release so that it is closer to zero order, and alleviating the problem of initial burst. Applying a Landau transformation, the present moving boundary problem is transformed in to a fixed boundary one, and the resultant problem is solved numerically by a finite difference scheme. We show that assuming drug release occurs under a pseudosteady-state condition is appropriate if the ratio (inner radius of device/outer radius of device) is small and/or the mass transfer resistance of the membrane is large. The applicability of two kinds of pseudosteady-state assumptions is discussed. The performance of the pseudosteady-state based on the rate of release is better than that based on the moving front. The region of the present device where the rate of release remains constant increases with the decrease in the ratio (inner radius of device/outer radius of device) and/or the increase in the mass transfer resistance of the membrane. Also, the higher the degree of overloading the longer the device is capable of maintaining a zero-order release rate. (c) 2005 Elsevier Ltd. All rights reserved.