Absorbable gelatin sponge, commonly used in otosurgery, was mixed with high-molecular-weight, highly concentrated hyaluronic acid. The mixture was introduced into the middle ear cavities of rats. The postoperative changes were evaluated by morphologic techniques after two months and three months. The middle ear cavity contained a loose mass of connective tissue with few cells, forming sail-like bridles between air-filled spaces. The filling material was easily detached from the surroundings. The structure of the middle ear mucosa and the tympanic membrane was well preserved. Compared with the findings in a previous study, in which absorbable gelatin sponge alone was used, the combination of gelatin sponge and hyaluronic acid caused significantly less structural alterations in the middle ear cavity. It is concluded that hyaluronic acid, a natural component in most tissues, markedly reduces the formation of the fibrous connective tissue known to develop in middle ear cavities filled with absorbable gelatin sponge alone.