1. Total body potassium was measured in professional football players before the football season began and at approximately mid-season. The values obtained were significantly higher than in healthy subjects taking only average exercise. There was a significant increase in body potassium as the season progressed. 2. Lean body mass was estimated by four methods and the results were compared. The values of lean body mass, as a percentage of body weight, were significantly higher than in healthy control subjects taking only average exercise and they also increased significantly over the period of the study. 3. The mean erythrocyte potassium concentration was lower pre-season than at mid-season and the pre-season value was also significantly less than in healthy controls. The values before and after vigorous exercise were not significantly different. 4. Conversely, the pre-season mean plasma potassium concentration was significantly higher than in control subjects and than the mid-season value. The mean plasma potassium concentration was higher before vigorous exercise than immediately afterwards. 5. The leucocyte count increased significantly after vigorous exercise, correlating with a decrease in leucocyte ascorbic acid content.