In well-trained middle-aged (45-55 years) athletes, O2 uptake, cardiac output (dye-dilution technique), heart rate, and arterial blood pressure were determined at rest in the supine and sitting positions, and during sub-maximal and maximal exercise in the sitting position. The heart volume was measured at rest (prone). The maximal O2 uptake was 3. 56/min and the maximal cardiac output 26. 8/min. The stroke volume was 19% lower at rest supine than during exercise and reached an average maximal value of 163 ml. The relation between maximal stroke volume and heart volume does not differ from what is found in young individuals. The arteriovenous O2 difference was 45 ml/1 at rest supine, but increased only to 133 ml/1 during maximal exercise. The low arteriovenous O2 difference seems to De the main limiting factor for the O2 uptake and might be explained by the relatively low hemoglobin concentration combined with peripheral factors.