If instillation of a local anesthetic agent into external auditory canal easily controls the pain, we can avoid the use of general anesthesia or invasive infiltration techniques for myringotomy and insertion of a tympanostomy tube. But topical anesthetics alone can't easily penetrate the tympanic membrane because the outer layer consists of squamous epithelium. So powerful solvent such as dimethyl sulfoxide is needed to let topical anesthetics absorb. The purpose of this study was to investigate morphologically the effect of tetracaine dissolved in dimethyl sulfoxide(DMSO) on the tympanic membrane of the guinea pigs. The change of tympanic membranes was studied by transmission electron microscopy. Thirty six healthy guinea pigs, exhibiting normal external auditory canal and tympanic membrane, were divided into two groups. Each group consisted of eighteen guinea pigs treated with 5% tetracaine in 95% DMSO solution or 95% DMSO alone. The animals in each group were sacrificed at 4 hours, 1 day, 3 days, 7 days, 2 weeks and 4 weeks after the application of the agents respectively. The histological changes were, in gerneral, rather similar for both agents, although certain differences were noted. The healing process was characterized by hyperplasia of the connective tissue layers of the drum. However, the most prominent feature of this process was the hyperplastic epithelium. Tetracaine base in DMSO caused a loss of epithelium and mucosal cells in 3 days. Regeneration started within 7 days in drums treated with tetracaine base or DMSO alone. As a result, it may be concluded that an transient inflammatory reaction is obvious when using these clinically potent agents, but it may not leave irreversible structural changes.