OBJECTIVE: The purpose of study was to demonstrate the presence of motor input from the spinal accessory and the branches of the upper cervical plexus. METHOD: Twenty-four patients were studied during modified radical neck dissection. The entire length of the spinal accessory nerve, the contributions from the upper cervical plexus and some cervical plexus branches to run to the trapezius independently were preserved in each of these patients. Compound muscle action potentials were measured to each part of the trapezius muscle on stimulation of the spinal accessory, C2, C3, and C4 nerves. RESULTS: Spinal motor nerve evoked responses were obtained from all 24 patients in the upper, middle, and lower trapezius. C2 contributions were seen in 2 out of 24 patients, but were in no patient supplying all three parts of the muscle. C3 contributions were seen in 11 out of 24 patients, but C3 nerve supplied all three parts of the muscle in 8. C4 contributions were seen in 20 out of 24 patients, supplying all three parts of the muscle in 16. CONCLUSION: This study demonstrated that the spinal accessory nerve provided the most important and consistent motor input to the trapezius muscle. Although C2, C3, and C4 provided motor input to the trapezius muscle, they were not consistently present and if present, did not consistently innervate all three parts of the trapezius. Compared with other studies, it was interesting to note that C4 gave more consistent motor input to the trapezius than other cervical branches.