The aminoglycoside antibiotics gentamicin and tobramycin were given to six healthy volunteers by intravenous injection. A dose of 80 mg was injected within 1 min. The curves relating serum concentration to time did not differ significantly for the two drugs. The maximal serum concentrations of drug exceeded 10 mug/ml on seven of 12 occasions. Despite these high concentrations, loss of hearing was not significant at any frequency (1,000-8,000 Hz) up to one month after injection. It is suggested that the aminoglycoside antibiotics penetrate into the inner ear lymph very slowly and that the concentration at that site is relatively unaffected by transient high concentrations of antibiotic in the blood plasma.