The potential ototoxicity of netilmicin and amikacin was compared in guinea pigs. When the toxicity in animals is determined two parameters should be discussed: Dose related comparison of ototoxicity and a comparison related to the antibacterial potency. None of the netilmicin test groups (50-100 mg/kg body weight) showed a higher percentage of degenerated hair cells than the control group. Even the individual animals showed a sensory cell damage within the control animal range. With the antibacterial potency taken into account, a dose three times higher was given to the animals treated with amikacin than to those treated with netilmicin. The amikacin group of animals showed a high percentage of degenerated hair cells under these conditions.