The data reviewed suggest that: 1. The BG are not only concerned with motor functions. 2. The BG are not directly involved in the control of neurophysiological, behavioral or homeostatic functions at a primary, elementary level. Thus, the effects of total ablation of the main component of the system, i,e. the caudate nuclei, demonstrates that the BG are not indispensable for life consciousness or the basic elementary integration of movements or sensory processes. 3. The BG operate at a high level of CNS integration and appears to be involved in two main types of, generally speaking, sensorimotor functions: a) The control of some of the organism-environment inter-relationships, both at a behavioral and neurological levels, a context of regulating the balance between approach and avoidance reactions. Some of the features of the acaudate cats suggest that this regulation might also include affective type reactions. b) The preparation or "setting up" of the organism for performance of both complex motor responses (response set), and of task requiring a high level of cognition (cognitive set). 26 High level of integration means here that, in the above functions, the BG control most probably operates upon performances not triggered reflexy, directly or indirectly, from the periphery but originated internally either "volitionally" or generated by symbolic, e.g. verbal, instructions. 4. The above functions appears to be accomplished by means of a modulatory action upon afferent signals arriving into the telencephalon and triggering efferent activities through forebrain output structures, particularly the neocortex. In normal conditions such modulation is seemingly carried on by means of a selective, flexible play of the intrinsic inhibitory mechanisms of the BG. When such control is disturbed either by pathology or by experimental manipulations, abnormal functional manifestations occur. These can be understood along the general concepts of (a) "release" from the BG inhibitory control ("compulsory approaching", hyperactivity, hyper reactivity, involuntary movements, abnormal postures, rigidity) or (b) "deficit" of the "setting up" for action postulated as a positive effect of striatal modulation (akinesia). The latter is viewed, therefore, as a permissive effect of the modulation, i.e., by selective removal of the inhibition, action is allowed to go through and to be expressed in actual performance. 5. Since lesions to individual BG structures, produced either neurosurgically in man or experimentally in animals, appear not to be capable of reproducing the complete clinical manifestations of any of the BG diseases, it follows that most of the BG syndromes in man must result from involvement of several BG components and often of other brain areas as well. More experimental work using the multiple lesions approach is needed to further ascertain this statement. 6. The literature on the effect of lesions and stimulation experiments, in particular, suggest that the BG are also involved in mental processes...