Propylene glycol, previously indicated to be ototoxic and to produce deafness, was studied in the middle ear space of guinea pigs. Propylene glycol (90%) produced conductive middle ear problems, but there was no loss of hair cells above that found in normal untreated animals. Propylene glycol (10%) produced no negative effects in the middle ear or elsewhere. Topical ear drops containing high concentrations of propylene glycol are contraindicated in cases with perforation of the tympanic membrane.