Exercise-induced muscle cramp has been considered to result from disturbances of fluid and electrolyte balance resulting from excessive sweat loss. Serum biochemical and haematological measurements were made on 82 male marathon runners before and after a 42.2-km race. Fifteen (18%) of the runners reported an attack of muscle cramp which occurred after 35 +/- 6 km (mean +/- S.D.) had been covered. These subjects were not different from the others in terms of racing performance or training status. Serum electrolyte concentrations, including sodium and potassium, were not different between those suffering from cramp and those not so affected either before or after the race, although a significant (P less than 0.001) increase in serum sodium concentrations occurred in both groups. Serum bicarbonate concentrations fell to the same extent (from 28 to 24 mmol l-1) in both groups. Significant decreases in plasma volume, calculated from the changes in circulating haemoglobin and haematocrit, occurred in both groups of subjects, but there was no difference in the extent of the haemoconcentration. The results suggest that exercise-induced muscle cramp may not be associated with gross disturbances of fluid and electrolyte balance.