The cochlear potentials were studied in rhesus and squirrel monkeys. Contrary to previous reports, the data revealed no important differences between the results obtained in these animals and those reported for the cat and guinea pig. Round-window recording of responses to sound stimuli exhibited cochlear microphonics of nearly 2 mV at maximum. The input-output curves showed that CM of these primates behaved not differently from those reported for the cat and guinea pig. When a micropipette was advanced from scala tympani into the organ of Corti, an increase in magnitude of CM was recorded, as well as a negative dc potential of about 75 mV. Perforation of the reticular lamina was associated with a change in polarity of CM and the appearance of an endocochlear potential (about +75 mV). Oxygen deprivation depressed CM and changed the polarity of the endocochlear potential from +75 to −20 mV. A comparative anatomical study showed that the cochleas of primates, cat, and guinea pig are similar.