Changing the pH of the dissolution medium has been found to affect the release of imipramine from gel-precipitated aluminium hydroxide spheres. Release from unwashed, unheated spheres into solutions of pH 1.2 was controlled by dissolution of the gel matrix, whereas that into solutions of pH 3 and pH 5 appeared to be under diffusion control. The liberation of drug from unwashed, heated spheres into the media of higher pH exhibited more complex kinetics. Washed spheres failed to release significant amounts of imipramine into the solutions of pH 3 and 5. Changing the ionic strength of the media had little effect on drug release. These phenomena have been explained with reference to model theories of the precipitation and ageing of aluminum hydroxide gels and their pH-solubility profiles.