Interleukin (IL)-6 is a pleiotropic cytokine, which has a variety of physiological roles including functions within the central nervous system. Circulating IL-6 increases markedly during exercise, partly due to the release of IL-6 from the contracting skeletal muscles, and exercise-induced IL-6 may be linked with central fatigue, which is enhanced by hyperthermia. Exercise-induced IL-6 may also stimulate hepatic glycogenolysis, which is important during prolonged and repeated exercise. Thus, in a randomised order and separated by 60 min of rest, eight young male subjects completed two 60 min exercise bouts: one bout with a normal (38 degrees C) and the other with an elevated (39.5 degrees C) core temperature. The cerebral IL-6 response was determined on the basis of internal jugular venous to arterial IL-6 differences and global cerebral blood flow. There was no net release or uptake of IL-6 in the brain at rest or after 15 min of exercise, but a small release of IL-6 was observed after 60 min of exercise in the first bout (0.06 +/- 0.03 ng min(-1)). This release of IL-6 from the brain was five-fold greater at the end of the second bout (0.30 +/- 0.08 ng min(-1); P < 0.05) with no separate influence of hyperthermia. In conclusion, IL-6 is released from the brain during prolonged exercise in humans and it appears that the duration of the exercise rather than the increase in body temperature dictates the cerebral IL-6 response.