The distribution of AchE activity in the cochleae of 42 normal guinea pigs was studied with thiocholine Cu-ferrocyanide method. Light microscopy revealed the AchE activity in the following areas: 1. the inner spiral bundle, 2. the spiral tunnel bundle, 3. the upper tunnel radial fibers, and 4. the large nerve endings at the base of the outer hair cells. Activity of AchE was most abundant in the basal turn and decreased gradually towards the apex. There was also a decreasing AchE from the first to the third row of outer hair cells. In the fourth turn, AchE activity was demonstrated only in the large nerve endings under the first row of outer hair cells. Under electron microscopy, the reaction was found to be localized to the following structures: 1. the axolemma of the small efferent fibers and their presynaptic enlargements in the inner spiral bundle, 2. the outer surface of the upper tunnel radial fibers and the spiral tunnel bundle, and 3. the large efferent nerve endings in contact with the base of the outer hair cells.