The present study investigated the effect of 7 days of strenuous exercise on the quantity of xanthine oxidase and IGF-I in muscle. Fifteen male military trainees performed 1 week of terrain marching and warfare exercises. Muscle biopsies and blood samples were obtained prior to and after the week. After the week, the number of xanthine oxidase immunoreactive cells, identified as capillary endothelial cells and leucocytes, and the number of IGF-I immunoreactive cells, mainly vascular cells but also cells tentatively identified as satellite cells, were higher in the muscle (P < 0.05). Plasma creatine kinase activity was 650% higher after the week (P < 0.001) and the muscle content of hydroxyproline was elevated by 160% 2 months post-exercise (P < 0.05), both findings implying injury to the muscle. The present data provide a first observation of an elevated level of xanthine oxidase and IGF-I in human skeletal muscle after exercise. It is proposed that both substances increased as a result of cellular damage: xanthine oxidase because of the influence of immunomodulators, and IGF-I in association with regenerative processes. The increased expression of IGF-I in the muscle could, however, also reflect cellular growth in response to an elevated load on the muscle and the vascular bed.