The long-term behaviour of cellulose sponge implants, 10 x 10 x 5 mm in size, and tissue reactions in and around them were examined in the subcutaneous tissue of the rat from 1 to 60 weeks after implantation. The cellulose sponge used was filled up with connective tissue 4 to 8 weeks after implantation. Histologically, moderate foreign body tissue reaction inside the implant, the appearance of cracks and fissures, spotty colouration, and softening of the pore walls were observed up to 16 weeks after implantation. Later, the foreign body reaction inside the sponge became milder, the spotty colouration disappeared and micropores enlarged in the viscose cellulose matrix. Histomorphometrically, the cross-sectional area of the implants and the size of the pore wall fragments decreased, and the number of pore wall fragments increased significantly. The cellulose sponge used can be regarded as a slowly degradable implantation material. However, the time needed for the total disappearance of the cellulose sponge from subcutaneous tissue is longer than the 60 weeks.