Plasma interleukin-6 (IL-6) concentration has been shown to increase with exercise and various cell types and tissues have been suggested to be responsible for this increase. At present no studies have measured the interstitial concentration of IL-6 in skeletal muscle and connective tissue. The present study represents the first attempt to simultaneously measure IL-6 in plasma, skeletal muscle and peritendinous connective tissue in response to prolonged exercise. Six healthy well-trained volunteers completed a 36 km run (flat, 12 km h(-1)). IL-6 was measured before, 2 h post-exercise and 24 h, 48 h, 72 h and 96 h post-exercise in both the medial gastrocnemius muscle (not measured at rest due to risk of disabling the subsequent exercise, and 24 h and 72 h post-exercise) and the peritendinous tissue around the Achilles tendon using microdialysis catheters with a high molecular mass cut-off value (3000 kDa). The plasma concentration of IL-6 was measured simultaneously, and in addition every hour during the exercise, by enzyme-linked immunosorbent assay (ELISA). The plasma concentration of IL-6 was found to increase throughout the exercise, reaching peak values immediately after completion of the run (50-fold increase). Using the microdialysis technique, the interstitial concentration of IL-6 was found to increase dramatically from 0 +/- 0 pg ml(-1) to 3618 +/- 1239 pg ml(-1) in the peritendinous tissue in the hours following the exercise. The pattern of changes was similar in plasma and peritendinous tissue, although approximately 100-fold higher in the latter. For comparison the interstitial muscle concentration was found to be 465 +/- 176 pg ml(-1) when measured 2 h post-exercise and 223 +/- 113 pg ml(-1) and 198 +/- 96 pg ml(-1) 48 h and 96 h post-exercise, respectively. The present study demonstrates that the connective tissue around the human Achilles tendon produces significant amounts of IL-6 in response to prolonged physical activity, which might contribute to the exercise-induced increase in IL-6 found in plasma.