The CSF of twenty-six patients with multiple sclerosis (MS) and of twelve control persons was analysed for histamine and histamine-N-methyltransferase (HMT) activity. Both in patients with remitting and progressive type of disease the histamine level was about 60% higher than in the controls. On the other hand in both groups of patients the HMT activity was significantly lower (32% and 40% respectively) than that of controls. These results suggest an altered histamine metabolism in MS. This might be associated with the immune reaction in MS brain.