Alginates, naturally derived linear copolymers of 1,4-linked beta-D-mannuronic acid and alpha-L-guluronic acid residues, can form hydrogels in the presence of divalent cations. In this study alginate hydrogels were formed by the addition of Ca2+ ions to aqueous solutions of sodium alginate. The rheological and mechanical behaviors of the hydrogels were studied using an axisymmetric probe tack apparatus with stress relaxation and cyclic movement capabilities. The hydrogels were subjected to high compressive strains (several units) to elucidate the source of nonelastic stress relaxation. These polysaccharide hydrogels behave elastically at small strains and become viscoelastic at large strains, a result that is responsible for the excellent toughness of the gels.