Abstract—Choline acetyltransferase (ChAc; EC 2.3.1.6) was assayed in the membranous cochlea and in the eighth cranial nerve (both the vestibular and cochlear components) from the point where it leaves the brain stem to the internal auditory meatus of the cat. To determine the contribution of the efferent innervation of the cochlea to this enzymatic activity both eighth nerves and both membranous cochleae were assayed at 17–29 days following section of the right, crossed and uncrossed olivo‐cochlear bundles (OCB) in the cat. The lesion was produced along the right sulcus limitans on the floor of the fourth ventricle. The left eighth nerves and cochleae served as controls in the ChAc assay. There was a significant decrease in ChAc activity in the right cochlea and eighth nerve after OCB section and degeneration. The mean activity of ChAc in the right cochleae of the 6 operated cats was 15 ± 7 μg of ACh formed. h−1(g wet wt. of tissue)−1in comparison to the rate of all the intact cochleae of 156 ± 38 μg of ACh formed. h−1. (g of tissue)−1, a statistically significant difference (P<0005). The mean activity of ChAc in the right eighth nerves of the cats with OCB lesions was 30 ± 8 n‐g of ACh formed. h−1. (g of tissue)−1in comparison to 91 ± 19 fig of ACh formed . h−1. (g of tissue)−1found for intact eighth nerves. This difference was also significant (P<0005). Thus, section and degeneration of the crossed and uncrossed OCB reduce the activity of ChAc in the eighth nerve and membranous cochlea. This finding provides support for the hypothesis suggesting the cholinergic nature o