The ototoxicity of chloramphenicol ear drops was studied in guinea-pigs. The ear drops consisted of chloramphenicol sodium succinate in propylene glycol; hence the ototoxicity of each chemical was also studied. The diluted chemicals were instilled in the middle ear cavity; thus clinical conditions were simulated, and the cochlear microphonic responses from the electrode on the round window of the cochlea were monitored as an indication of the hair cell function. Our experiment revealed that propylene glycol at the concentration of 10% or more caused irreversible deafness. Chloramphenicol sodium succinate in a concentration of 5% or more in Ringer's solution also caused irreversible deafness. It is recommended that propylene glycol should not be used in the ear drops as a solvent of the drug. Chloramphenicol at higher concentrations is not recommended for use in the middle ear cavity.