Abstract Influence of ethacrynic acid (EA) upon gentamicin kinetics in perilymph and endolymph was studied in rats that were given a constant-infusion of gentamicin (150 μg/min) and EA (140 μg/min). Inner ear fluids and plasma were sampled up to 5 h. The purity of the endolymph was ensured by measurement of sodium and potassium concentrations. Gentamicin assay was done with a modified radioimmunoassay. Results show that EA facilitates the entry of gentamicin into endolymph, while it does not affect the kinetics of the drug in perilymph. Although the mechanism of this facilitation remains unclear, this result may account for the ototoxic potentiation reported between EA and aminoglycoside antibiotics.