Twenty-three football players were studied before and after a match to assess if intermittent exertion produced the same biochemical and physiological changes noted in marathon runners. All players developed a significant fluid deficit and rise in body temperature despite ready access to water. However, the changes were less marked than those described in runners. They also showed acidosis, a rise in serum levels of creatinine, urate and urea and striking elevations in the levels of creatine kinase and other enzymes. Serum magnesium levels fell, as did those of potassium; this is not the case in runners, in whom hyperkalaemia is more frequently described.