An exact solution has been obtained for the release kinetics of a solute from a rectangular reservoir into a finite external volume. The exact solution is derived based on the Laplace transform method. Comparisons between this new solution and previously reported approximate solution based on pseudo-steady-state assumption indicate that the approximate solution is very accurate at late time but at early time deviations from the exact solution increase. The results presented here indicate that as the external fluid volume increases, the cumulative release at any time and the releasable amount of the solute at infinite time increase.