Abstract: After administration of a tablet containing both sodium alginate and soluble calcium salts, an acid insoluble calcium alginate gel matrix can be formed immediately upon wetting with gastric fluid (Fig 1). The active ingredient contained therein is released gradually and a prolonged-action of the drug can be produced accordingly.Scanning electron micrographs were taken for better understanding the structure of the matrix (Fig2, Fig3). It was found that all the particle size of the sodium alginate powder, the forms and quantities of calcium salts, the tabletting processes and the sizes of the dried granules to be compressed etc. affected the release rate markedly. Several drugs including isosorbide dinitrate, procainamide, phenytoin, chloropromazine, and theophylline were incorporated into the alginate formulations separately. The dissolution rates obtained revealed that sustained release effects were achieved to some degree by this type of tablets (Fig4).In considering the relative short half-life characterized by isosorbide dinitrate (ISDN), efforts were made to develop a satisfactory formulation with a suitable dissolution rate. A formulation with t₃₀ about 50 min. was obtained by using fine powder of sodium alginate (120 mesh) and medium size granules.