In 10 healthy subjects stimulation of the accessory nerve in the posterior triangle of the neck always elicited action potentials in all 3 parts of the trapezius muscle. Of 8 patients with a lesion of the accessory nerve 3 had no evoked potentials, while 5 showed evoked potentials in the 3 parts of the muscle. 7 of these patients had voluntary activity in the lower part of the trapezius muscle. 7 patients with lesions of the accessory nerve showed, by electromyographic investigation, no constant pattern of the neurogenic involvement in the 3 different parts of the muscle. Direct stimulation of the accessory nerve during neck dissection in 3 patients gave evoked potentials in the ascending and the horizontal part in all 3 cases and in the descending part in 2 cases.