The etiology of poststapedectomy reparative granuloma remains obscure. To evaluate the possibility of inadvertently introduced mucous membrane into the vestibule as a possible cause of granuloma formation, this animal study was devised. The stapes was removed from the right ear of four squirrel monkeys and middle ear mucosa placed in the vistibule, the stapes then being replaced. In the left ear, a control operation was done in which only the stapes was removed, then replaced. The animals were sacrificed between the 2nd and 17th day and their temporal bones were prepared and studied. The mucous membrane was identified within the vestibule of all animals. It was well-tolerated and did not evoke the inflammatory response, which would be expected if mucosa was a cause of granuloma formation. The mucosa was still present at 17 days, eventually becoming surrounded by the endosteal lining of the vestibule.