Distribution of the trophic peptide somatomedin C (Sm-C; insulin-like growth factor I; IGF-I) immunoreactivity was mapped in normal Achilles and tibialis anterior tendons. The spindle-shaped tendon fibroblasts showed faint perinuclear staining. Fibroblasts in the paratenon mostly had a more intense IGF-I immunoreactivity, i.e. faint to moderate. When analysing either tendon in detail, areas with more intense IGF-I immunoreactivity could be recognized and seemed to correlate with areas of high mechanical stress. Increased mechanical load induced over 3 days elevated IGF-I immunoreactivity throughout the cytoplasm of tendon fibroblasts. Peak intensity was reached in 7 days, and thereafter the IGF-I immunoreactivity seemed to decrease irrespective of persistent high mechanical load. Training the animals on a treadmill for from 20 up to 60 min per day for 5 days induced after 3-5 days increased IGF-I immunoreactivity throughout the cytoplasm of the tendon and paratenon fibroblasts. Sudden curtailment of loading the Achilles tendon resulted in a marked reduction of the IGF-I immunoreactivity in most fibroblasts within 3 days. After a week only a small number of tendon fibroblasts showed any IGF-I immunoreactivity. The IGF-I immunoreactivity of tendon fibroblasts thus correlates to mechanical loading of the tendon. It is proposed that IGF-I may have a trophic influence on tendon and paratenon cells by autocrine and/or paracrine mechanisms.