A number of biochemical and haematological parameters, including plasma electrolytes, parameters of hepatic and renal function, plasma enzymes and free fatty acids were measured in 13 athletes before and after a 160-km 24-hour race. The runners were divided into 2 groups: group A, who competed the 160 km within 24 hours and group B, who either ran for 24 hours, or who retired before completing the distance. Minimal changes were found in the plasma electrolyte patterns in either group, whereas blood urea and creatinine levels increased during the race. The plasma enzymes increased to varying extents, the greatest increases being in lactic dehydrogenase, aspartate aminotransferase and the skeletal muscle specific MM isoenzyme of creatinine phosphokinase. Total bilirubin also increased, but no conclusive evidence of hepatic decompensation was found. Plasma free fatty acids levels were very markedly raised in 12 of the runners, the highest increases occurring in group A. All runners ingested carbohydrate during the race and this probably explains why the blood glucose levels increased slightly but remained within normal limits in all the athletes at the end of the race.