Objective. To determine the mitochondrial dysfunction in multiple sclerosis (MS). Material and methods. Fourteen adult patients with MS and 23 healthy people were examined. Cytochemical analysis of lymphocytes in peripheral blood was carried out. The activity of the mitochondrial enzymes involved in metabolism of carbohydrates (lactate dehydrogenase, LDH), amino acids (glutamate dehydrogenase, GDH), fatty acids (alpha-glycerophosphate, α-GPDH) and II complex of the mitochondrial respiratory chain (succinate dehydrogenase, LDH), and the level of lactate in the blood were measured. Results and conclusion. The activity of α-GPDH was reduced in 62.5% patients and increased in 37.5%. GDH activity in all patients was lowered. LDH activity was reduced in 71.4% patients and compensatory increased in 28.6%. In 66.7% patients, LDH activity was reduced and in 33.3% compensatory increased. Increased blood lactate was observed in 33.3% patients before meal and 44.4% after carbohydrate loading. After carbohydrate loading, in 33.3% patients lactate levels in the blood increased and in 11.1% increased above normal values. Therefore, the enzyme activity have decreased in most patients that suggests the decompensation of mitochondrial function. The results indicate the advisability of administering the energotrophic drugs carnicetin and coenzyme Q10 (idebenone) in patients with MS.