Spasticity results from various pathophysiologic abnormalities in spinal cord neuronal circuits. Noninvasive electrophysiologic techniques can be used to study these circuits in humans. The best correlation between briskness of reflexes and results of electrophysiologic testing is found with reduction in vibratory inhibition, a test that reflects presynaptic inhibition. For increase in muscle tone, the best correlation is found with reduction of Ib nonreciprocal inhibition. These test results, which are stable under controlled conditions, are influenced by myorelaxant drugs and may be used to analyze the mode of action of new products because the tests study specific circuits involving known neurotransmitters. Tizanidine reinforces presynaptic inhibition and two types of postsynaptic inhibition: Ia reciprocal and Ib nonreciprocal. It also markedly reduces flexor reflexes. These effects are explained by an action exerted on spinal interneurons deprived of their normal monoaminergic descending innervation. The spectrum of activity for tizanidine is thus broad, making it likely that tizanidine corrects more than one pathophysiologic abnormality. Because tizanidine reinforces presynaptic as well as Ib nonreciprocal inhibition, it may reduce both brisk tendon jerks and muscle hypertonia.