Reaction times (RT) were measured in 39 multiple sclerosis (MS) patients and 25 healthy controls, before and after comprehensive neuropsychological assessment lasting at least 4 h. The assessment required prolonged mental effort, which was assumed to induce fatigue. Simple and disjunctive RT were measured for visual and auditory stimuli. RT before and after prolonged effort were significantly longer in MS than in control subjects (with exception of auditory RT). This effect was related to disease duration and to simultaneous presence of brainstem, cerebellar and/or pyramidal signs. Age did not significantly contribute to the explanation of RT increase. In female patients, increasing distribution of functional lesions in the central nervous system was related with greater RT increase than in male patients. RT of MS and controls changed after prolonged mental effort and effort-related fatigue was experienced subjectively. Yet no differences in direction or degree of change in RT were found between controls and ambulant patients with stable MS who apparently had resources to overcome fatigue.