The effects of neomycin on tissue levels of ethacrynic acid was studied in vivo and in vitro. In vitro, the addition of neomycin to incubations of synaptosomes and inner ear tissues of guinea pigs raised the concentration of bound ethacrynate approximately two-fold. In vivo, radioactive ethacrynate was determined in the inner ear of mice by radioautography after intravenous injection of the drug (50 mg/kg body weight). Treatment of animals with neomycin (100 mg/kg body weight, intramuscularly) 60 min prior to ethacrynate led to a three-to five-fold higher accumulation of ethacrynate in cochlear structures. It is suggested that neomycin breaks down hemolabyrinthine or tissue permeability barriers and allows increased penetration of the other drug into the inner ear.