1. The axonal conduction velocity and the voluntary discharge properties of 120 short toe extensor motor units were studied in man. 2. Electromyographic techniques were used which permitted the identification of individual motor unit potentials after proximal and distal electrical nerve stimulation and during maximum voluntary effort. 3. The necessary selectivity of the e.m.g. recordings was achieved in two circumstances. In some subjects, previous motor nerve lesions distal to the point of stimulation had led to collateral sprouting with larger motor unit potentials. In other subjects an accessory deep peroneal nerve was present, so that lidocaine block of the main motor nerve left a small number of innervated motor units. 4. The axonal conduction velocities of the individual motor units ranged from 30 to 54 m/sec with most motor units between 35 and 45 m/sec. 5. Motor units which voluntarily could be driven continuously at frequencies below 10/sec had axonal conduction velocities between 30 and 45 m/sec. 6. Motor units which on voluntary drive responded only in high frequency bursts had axonal conduction velocities between 40 and 54 m/sec. 7. Motor units with intermediate voluntary discharge properties had intermediate axonal conduction velocities. 8. Thus a relationship was established between voluntary discharge properties and axonal conduction velocity.