Two patients who had suffered severe carbon monoxide intoxication showed lasting neuropsychologic sequelae: 1) deep inertia involving the whole behaviour and expressed by an almost complete lack of activity if not induced by someone else; a mental gap when the patients were left to themselves and a tendency to give up the mental activity when stimuli ceased; an apparent affective indifference connected with a lack of spontaneous expression of the affects; 2) pseudo-obsessionnal activities: coprolalia with sexual themes in one patient, obsessive collecting and tidying up activities in the other one. Neurologic examination was normal, particularly no parkinsonian syndrome was present. CT scans showed bilateral pallidal low density areas. Both patients had moderate intellectual impairment in psychometric tests and amnestic disorders. There is a possible relationship between memory and cognitive impairment and mental inertia. The onset of pseudo-obsessional signs following basal ganglia lesions is emphasized. The central semeiological fact seems to be a disorder of the initiation and the carrying on of any external action as well as mental activity itself. This may be related with the activity disorders represented at a more elementary level by motion impairment in parkinsonian akinesia.