The ratio of the amplitudes of summating potential(SP) to action potential(AP), commonly referred to as the SP/AP ratio, has been suggested as a diagnostic tool for Meniere’s disease. It was confirmed in many studies that an abnormally large SP in Meniere’s disease causes an abnormally increased SP/AP ratio of electrocochleogram(ECoG). Recent studies in patients with Meniere’s disease and in animal models of endolymphatic hydrops have demonstrated a decrease in SP toward normal and a reduction of the SP/AP ratio after glycerol dehydration. However, the lack of control data for normal individuals makes it difficult to interpret small changes in SP/AP ratio in patients with endolymphatic hydrops. This study was, therefore, undertaken to evaluate changes in electrochochleogram of normal subjects in reponse to a dehydration induced by urea administration. Fourty five volunteers without otologic disease were tested three-times with extratympanic ECoG. The SP and AP were measured at a 95 dB HL of click stimulus. The results are as follows : 1) The SP/AP ratio was not significantly different between the male and female subjects both in the baseline and dehydration conditions. 2) There was no difference in the SP/AP ratio between the right and left ears of the same subject. 3) In the left ears of 35 males, the dehydration SP/AP ratio at 60 and 90 minutes after urea administration were lower than the baseline SP/AP ratio. 4) The SP/AP ratio tended to decrease as the serum osmolality increased in 24 males.