A new method for the quantitative assessment of acute ototoxic side effects of drugs in described. It is suitable for screening purposes. The method is based on the determination of the toxic dose (TD 50) which causes a defined hearing loss in 50% of the animals tested. The hearing loss is defined as a complete suppression of the compound action potential (CAP) of the auditory nerve, elicited by clicks 30 dB above threshold. This is approximately equivalent to a clinical hearing loss of 30 dB. The TD 50 is used to estimate the therapeutic range. With this approach ototoxic side effects of furosemide, piretanide and bumetanide were compared quantitatively in cats. The TD 50 values for CAP suppression were 18.37 mg/kg for furosemide; 4.29 mg/kg for piretanide and 2.21 mg/kg for bumetanide. As equipotent diuretic doses are 2.61 mg/kg for furosemide, 0.26 mg/kg for piretanide and 1.16 mg/kg for bumetanide, it appears that the relative ototoxicity is least for piretanide and highest for bumetanide. Plasma concentrations, determined initially and when recovery of CAP to 50% of control had occured, indicate that bumetanide may be more slowly eliminated from the cochlear spaces than furosemide and piretanide.