The author's interest in the motor system is traced over a period of 30 years. Early studies on the cat pyramidal tract proved to be of relevance to later work on the control of muscle tone by brain stem and cortical mechanisms. Application of physiological methods disclosed fundamental differences between the increased tone of Parkinson's disease, spasticity and decerebrate rigidity. These differences depend upon the extent to which dynamic and static stretch reflexes are released, and whether the control effects of flexor reflex afferents remain under the control of the brain stem (as in Parkinson's disease), are released (as in spasticity) or even reversed (as in decerebrate rigidity).