The effect of Arnica Montana on stiffness, restitution time and cell damage during hard physical exercise was evaluated in a randomized double blind trial during the Oslo Marathon. 36 participants were randomized: one group received Arnica 10(-30) five pills twice daily for five days, starting the day before the event. The other group received placebo pills. Blood tests were carried out before and immediately after finish, and after 48 and 72 hours. Stiffness was evaluated on a visual analogue scale after finish and on the next three days. There was no difference between the groups as regards ALAT, ASAT, GT, LD, creatinine, haptoglobin or magnesium. CK increased in both groups, but to a higher level in the placebo group. The difference was greatest on the second day (p = 0.07). A feeling of stiffness was more pronounced in the placebo group on all four occasions (p = 0.06 and 0.07 on day 2 and 3). There was no indication that Arnica reduced the time of restitution.