The round window membrane(RWM) is the most likely route of entry from the middle ear to the inner ear. Middle ear effusion contain high concentrations of prostaglandins(PGs) and leukotriens(LTs) and the ototoxicity of nonsteroidal anti-inflammatory drugs(NSAIDs) may be mediated by PGs in the inner ear. Studies of the permeability of the RWM to PGs are important not only because of the possible secondary effect of PGs in the middle ear to the inner ear but also because of the possible therapeutic application of exogenous PGs on the RWM to achieve high level of PGs in the inner ear for treatment or prevention of ototoxicity of NSAIDs. Using 13 chinchillas as experimental animal, sample of perilymph were taken after 0.5, 1, 2, and 4 hours after placing the ³H-PGE₂(10,000 CPM/5μl) over the RWM in group I, and 0.5, 1, and 2 hours after placing 6-Keto-PGF₁ α(500μg/μl) over the RWM in group II. Radioactivity of PGs in the perilymph was measured by scintillation counter and concentration of the 60-Keto-PGF₁ α was measured by radioimmunoassay. The results were as follow ; 1) The percent count per minute(CPM, mean±SE) of radioactivities of perilymph after application of ³H-PGE₂ on the RWM were 1.21±0.48% CPM in 0.5 hour and peaked at 1 hour(2.19±0.34% CPM) and plateaued at 2 hours(1.85±0.47% CPM) and maintained at similar level(1.84±0.50% CPM) in 4 hours. 2) The concentration(mean±SE) of PGs that cross over RWM after application of 6-Keto-PGF₁ α was 264.25±18.05ng/μl at 0.5 hour and peaked at 1 hour(371.00±79.45ng/μl) and decreased at 2 hours(186.02±124.48ng/μl).