We analyzed metabolic parameters in 11 volunteers after each segment of an endurance triathlon and, in a separate year, analyzed similar parameters in eight volunteers during 6 d of recovery following completion of an endurance triathlon. After the 2.4-mile ocean swim, serum lactate tripled, and albumin and muscle enzymes were increased. After the 112-mile bicycle ride, mild dehydration occurred, and muscle enzymes and uric acid levels increased markedly. Serum lactate was elevated over baseline but was lower then after the swim. After the 26.2-mile run, dehydration and muscle damage progressed; serum triglycerides dropped by 50%. Serum lactate remained elevated, but less than after either of the other segments. During recovery, muscle enzymes continued to rise and peaked (creatine phosphokinase on the day following the triathlon at 4920 +/- 685 U.ml-1; range 1321-16,746); creatine phosphokinase and lactate dehydrogenase remained significantly elevated at the end of the recovery period. Total protein and albumin decreased, suggesting alterations in their synthesis or their utilization for tissue repair. Serum cholesterol levels fell significantly until the 4th d. Serum triglycerides slowly increased to baseline over 4 d, suggesting their use as energy substrate during recovery. Thus, competition in an endurance triathlon causes skeletal muscle injury that appears early, increases as the triathlon progresses, and is still apparent even 6 d after completion of the triathlon. Changes in plasma proteins and lipid suggest that energy substrate utilization is shifted as the triathlon progresses and as the body repairs itself following completion of the event.