Biochemical variables have been measured in a group of volunteers during and after a long-distance run. Plasma glucose levels remained relatively constant and a significant decrease in plasma bicarbonate was noted. Plasma sodium, chloride, total protein, albumin and calcium showed significant increased of an order compatible with water losses occurring during the run. Plasma potassium, urea, creatinine, uric acid, phosphate and bilirubin all show much more marked and variable increases. The plasma enzymes alkaline phosphatase, lactate dehydrogenase, aspartate aminotransferase and creatine kinase likewise increased significantly throughout the run. Whilst most constituents showed a tendency to return to normal at 20-30 hours after the run, gross increases were observed for aspartate aminotransferase and creatine kinase.