Serum osmolality and serum sodium, potassium, glucose, free fatty acid, glycerol, lactate, pyruvate, triglyceride, insulin and growth hormone levels were measured in 40 male athletes immediately after they had completed the 90 km Comrades Marathon. These results were compared with control values measured at the same time of day in the same subjects 6 weeks later. When compared with control values, serum sodium, glucose, free fatty acid, glycerol, lactate, pyruvate and growth hormone levels were all significantly elevated after the race. Serum insulin levels were reduced, whereas serum potassium and triglyceride levels were unchanged. These results, which are essentially similar to those reported in shorter (42 km) and longer (160 km) distance races, provide important field data complementing those collected in the laboratory. They suggest that the overall metabolic response to marathon and ultramarathon running is not substantially different.