1. The discharge properties of single motor units during prolonged maximal voluntary effort have been studied using electromyographic recordings, mainly in the short big toe extensor muscle but also in the anterior tibial muscle.2. The required selectivity of the e.m.g. recordings was achieved in the short big toe extensor muscle after previous mechanical lesions to the terminal nerve twigs and muscle fibres and consequent collateral sprouting, and in the anterior tibial muscle with the use of a high impedance wire electrode.3. During the first few hundred milliseconds of sustained maximal effort the motor units fired at rates ranging from about 30-60 Hz, and the tension was the same as that obtained on electrical tetanization of the nerve to the toe extensor muscles above 50 Hz.4. During prolonged maximal effort the firing rates and the proportion of motor units firing successively decreased. Motor units initially firing at 30 Hz continued to fire tonically but at 15-20 Hz. Motor units initially firing at 60 Hz ceased to fire tonically but could still be made to discharge phasically. The period of time during which all motor units responded tonically could be increased from some seconds up to 20 sec by long-term training.5. Motor units with a limited endurance fired at a lower tension in the early than in the late stages of maintained contraction.6. It is suggested that motoneurones innervating slow twitch muscle fibres respond continuously to prolonged voluntary drive at rates sufficient for full fusion but that the threshold of motoneurones innervating fast twitch muscle fibres increases so that they finally mainly fire phasically thus protecting the peripheral excitation and contractile mechanisms from excessive exhaustion.