The effects of prolonged tobramycin administration (given in repeated injections over a 15-day period) on auditory and vestibular functions were studied in normal subjects, in patients with renal impairment, and in chronic nephritic patients undergoing hemodialysis. With the doses used in this study, the repeated administration of tobramycin resulted in blood accumulation only in the group of patients with renal impairment. In one single case, administration of tobramycin was followed by a transient aggravation of a pre-existing renal impairment. Cochlear and vestibular functions were evaluated before treatment and repeated during and after drug administration. In normal subjects, a dosage of 50 mg/8 h failed to produce cochlear and vestibular dysfunction; but with dosages of 75 mg/8 h and 100 mg/8 h, changes in vestibular reflectivity occurred frequently, mostly of the irritative type. Generally moderate, but quite often slight changes persisted (5 of 10 cases). They were not accompanied by auditory or vestibular clinical signs. In patients with impaired renal function and in those undergoing chronic hemodialysis, vestibular impairment is customary and most often of the deficiency type. Half of the cases still showed detectable changes on follow-up evaluation that was performed 10 days after discontinuation of the drug.