Tobramycin, gentamicin, sisomicin and amikacin are aminoglycoside antibiotics that are used clinically. A significant side effect of aminoglycoside antibiotics is the production of hearing loss. Our study was done to determine the relative ototoxic liability of these 4 drugs. All drugs were given in daily doses of 0, 50,100 or 150 mg/kg to guinea pigs subcutaneously for 4 weeks. In addition, 200 mg/kg was was given to the animals receiving amikacin and sisomicin. There were 10 animals in each dosage group. Seven animals were used for auditory study and 3 were used for pharmacokinetic study. Auditory damage was assessed by determining the Preyer pinna reflex, the ability of the cochlea to generate the AC cochlear potential, and the number of sensory hair cells missing from the organ of Corti. The concentration of aminoglycoside antibiotics is determined in the cochlear perilymph and plasma by a radioenzymatic assay. On a equal dose basis the ototoxic liability of gentamicin and sisomicin were very similar and tobramycin and amikacin being less ototoxic than the gentamicin and sisomicin. The pharmacokinetics of single 150 mg/gk doses of the drugs were similar. The exception was that tobramycin reached lower peak levels in plasma and perilymph and its time to peak level in perilymph was delayed relative to the other drugs. Analysis of plasma and perilymph following chronic administration revealed higher concentration of gentamicin and sisomicin than of tobramycin and amikacin.