The acute effects of a i.m. injection of interferon beta-1a on the secretion of the cytokines interleukin-6, tumor necrosis factor-alpha, soluble tumor necrosis factor receptor I, and interleukin-1 receptor antagonist and its relation to peripheral concentrations of adrenocorticotropic hormone (ACTH) and cortisol in patients with MS were investigated, as well as the influence of cotreatment with indomethacin on these parameters. After interferon beta injection we found an acute rise in plasma cytokine levels and an increase in plasma hormone concentrations, both of which were modulated by indomethacin comedication.