Recently many kinds of drugs were introduced and contributed to treat diseases, and meanwhile the unwanted results have increased after overuse or misuse of drugs. In ENT field we have met such iatrogenic disorders as hearing impairment after medication or rhinitis medicamentosa afer long-termed nasal dressing. The authors performed an experimental study with the repeated instillation of nasal drops into the nasal cavity of 252 white rats during a period of 24 hours to 8 weeks, and effects of various nasal drops on mucosal changes were correlated with histopathologic findings. Animals were divided in 6 groups by the various concentration of nasal drops (silver protein solution, naphazoline hydrochloride solution), each being again examined in relation to experimental durations of 24 hours, 1, 2, 4, 6 and 8 weeks. The followings were the results ; 1. A most reliable and advanced pathologic alteration of chemical rhinitis or rhinitis medicamentosa was found in the groups of animals that were instillated with naphazoline drops, being more apparent in cases of 0.1% solution administration than in those of 0.05% solution. 2. Silver protein group and silver protein-naphazline mixed group exhibited the inflammatory changes in lighter degree than naphazoline group. 3. The circulatory disturbance and early exudative phenomena started within 24 hours, but the mucosal damages were ascertained from 1 week group, and the principal histopathologic changes, were noticed in the respiratory mucosa whereas mild non-specific changes in the integumentary and olfactory parts. 4. Major histologic changes encountered in the respiratory mucosa included marked edema and congestion, increased infiltration of inflammatory cells, damages of ciliated epithelium including ulceration, goblet cell hyperplasia, metaplastic changes of epithelium, disruption of basement membrane, mucous gland change and mucosal atrophy. 5. Ciliated epithelial changes such as loss of cilia and desquamation showed a tendency to increase roughly relating to the experimental period, and goblet cell hyperplasia appeared an early manifestation as well as tissue reaction to the weak stimulation, but decreased in the phase of mucosal atrophy. 6. Irreversible and permanent changes were not recognized during a period of experimental weeks, and yet fibroblastic proliferation, mucosal atrophy and metaplastic mucosal alteration may suggest the process leading to atrophic rhinitis.