Lesions of the upper motor neuron cause: 1. Alterations in segmental reflex activity. For example increased tendon jerks and velocity dependent stretch reflexes ("spasticity"), clonus, the clasp knife response, release of flexion reflexes and extensor plantar reflexes. 2. Impaired ability to activate motoneurons rapidly and selectively. Voluntary movements may also be restrained by co-contraction of antagonists muscles, by segmental reflexes (enhanced during voluntary effort) or by contractures. A combination of these factors may impair overall functional ability. Segmental reflexes, voluntary power and overall functional abilities can be assessed using clinical scoring systems. Recordings of muscle length, tension and EMG offer more objective measures of reflex and voluntary activity and of overall functions such as locomotion, and can separate weakness from co-contraction, spasticity from contracture. Methods are now available for exploring individual (transmitter specific) segmental reflex pathways and descending pathways in man. Lesions of the upper motor neuron are complicated by secondary changes in segmental neurons. Segmental reflex activity and muscle mechanics depend on the immediate past history of events. These factors must be taken into account.