The terms rigidity and spasticity are considered to denote different states in clinical neurology. Liddell and Sherrington9pointed out that the stretch reflex in decerebrate cats consisted of 2 phases, one being the "phasic" component, which appeared only during the period of increasing muscle stretch, and the other the "tonic" or "postural" component, which existed during the continuously sustained stretch. In previous papers, we11,12have reported that these 2 components can be demonstrated in the so-called hypertonic states of "extrapyramidal" disease and that the tonic component is more markedly and selectively decreased by stereotaxic pallidotomy than the phasic one. This paper is an investigation of the neurophysiological mechanisms which underlie these 2 patterns of stretch reflex, with particular reference to their relationship to the globus pallidus. The concept of a gamma loop, proposed by Granit,3introduced a plausible physiological explanation of the rigid and spastic states. The