 The TCD examination will be repeated only in the patients with MS i.e. not in control subjects 4 h after the oral intake of 62.5 mg bosentan Tracleer when peak plasma concentrations are expected. This is a purely academic study and Bosentan will be purchased through the hospital pharmacy. The most common side effect of daily treatment with bosentan is hepatotoxicity. Adverse reactions to one single dose are very unlikely. The primary aim is to investigate the potential of TCD to evaluate changes in the cerebral circulation between patients with MS and controls. Bosentan is not the object of investigation in this study but can be considered as a Non Investigational Medicinal Product N.I.M.P.. Patients with MS will receive the product to increase CBF, an effect which has already been demonstrated in an earlier perfusion-weighted MRI study. 