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Controlled release formations of drugs were prepared with matrix materials of the natural polysaccharides sodium alginate and +/--carrageenan. The drugs used were sodium salicylate (anionic) and lidocaine-HCl (cationic). The release characteristics of the drugs were studied under different pH and compositions of aqueous media. The cationic drug (lidocaine-HCl) was released more slowly than the anionic one (sodium salicylate) because of the negative charge of the matrices. The release from both sodium alginate and +/--carrageenan tablet was retarded in acidic medium. At the same pH of the medium, the release into buffer solutions (containing metallic cations Na(+) and K(+), respectively) was slower than into HC1 solution.