Within the optic and acoustic ranges of perception, the reaction periods and selective reaction periods of 40 patients suffering from multiple sclerosis were compared with those of 40 healthy control persons. The reaction time and the selective reaction time of the patients were significantly longer (p<0.05) than those of the controls. The patients selective reactions were most prominently affected. The selective reactions of the patients were more significantly delayed within the range of acoustic perception than within the optic range (p<0.01). There is not enough material in this work to prove a correlation between the changes of reaction time or selective reaction time and the duration of the disease or the number of phases. In the discussion, reference is made to the changes of functional parameters of demyelinated nerve fibres. The disturbances found can be regarded as due to functional changes of demyelinated nerve fibres in CNS, particularly in the cerebral white matter.