Histochemical investigation of acetylcholinesterase in the organ of Corti has been carried out by means of the Karnovsky's method. Acetylcholinesterase is located on the bottom of outer hair cells, tunnel radial fibers (upper), tunnel spiral bundle and inner spiral bundle. No reacrion was found on the bottom of inner hair cells, lower tunnel radial fibers, outer spiral bundle and bottom of outer hair cells on the outer raw at upper turns. In the bottom of outer hair cells, only vesiculated nerve endings showed the activity of acetylcholinesterase on their plasma membrane. In the basal and middle turns, it is assumed that the reaction products or enzyme from vesiculated nerve endings diffused to non-vesiculated nerve endings.On the endolymphatic surface of hair cells, acetylcholinesterase activity was also found but was lower than the activity on the vesiculated nerve endings at the bottom of the hair cells. In addition, some reductive reaction was found on the surface of the hair cells.