1. Elevated levels of cytokines, especially interleukin (IL)_6 and IL_1ra, can be measured in the plasma of athletes after exhaustive long term exercise. 2. The present study investigates the kinetics of several cytokines and chemokines in ten male athletes before, during and after 2·5 h of treadmill running at 75% of maximal oxygen consumption (VOµ,max). Blood was sampled before, every half-hour during running and every hour in the following 6 h recovery period. 3. The plasma concentration of IL_6 increased after 30 min of running, and peaked at the end of running with a 25-fold increase compared with the pre-exercise value. IL_1ra increased only after running, and peaked after 2 h of rest with an 18-fold increase compared with the pre-exercise value. No changes were found in the concentrations of IL_1â, tumour necrosis factor (TNF)á, IL_15 and macrophage inflammatory protein (MIP)_1â, and the concentrations of IL_8 and MIP_1á were below detection limits. 4. The results suggest that very early events in exercise trigger the release of IL_6, and that the cytokine response to exercise has similarities to that observed after trauma.