After a short historical remark the development of athlete's heart in childhood is described. Within 2 years significant differences were observed between endurance-trained (swimming) and untrained girls and boys determined by X-ray and echocardiographical examinations. The limits of the physiological size in relation to body weight were not exceeded within 10 years of longitudinal studies. A second point deals with athlete's heart from physiological and clinical viewpoints. The largest healthy heart ever found in our examinations of athletes had a size of 1,700 ml. Sixteen years after stopping the active career it was reduced to 950 ml without a pathological finding. Questionable and pathological cases are described. A third chapter covers the blood supply of internal organs during exercise combined with air or oxygen breathing. In this connection liver, kidneys, heart, lungs, and brain have been investigated. The reduced blood supply of the liver and kidneys during intense exercise on the cycle ergometer was not influenced significantly by inspiration of oxygen. A significant blood volume increase of the lungs was noticed during incremental rates of work. Exercise augmented also blood flow of the brain in relation to the work rate (at 100 W a 27% increase in grey matter flow of the right hemisphere). A fourth chapter deals with new hormonal and neurohormonal aspects related to the cardiovascular system. Beta-endorphines remained unchanged at work rates below the anaerobic threshold but increased significantly during maximal rate of work. The opiate antagonist naloxone abolished the rise in body temperature seen during ergometer exercise. The serotonin antagonist ketanserin lowered the blood pressure and the arterial lactic acid level during an incremental exercise test, similar to the results with the dopamine agonist pergolide. The hormone cardiodilatin is produced in the atrial appendages, and it is a potent substance in the regulation of the cardiovascular system. The adaptive reaction of the sympathetic nerve fibres in the myocard revealed different directions: activation, degeneration, and regeneration. These findings correlated highly significantly with the total amount of catecholamines in the heart muscle.