Plasma volume expansion usually occurs with acute endurance exercise and endurance training both in humans and in animals. In most cases, the increase in plasma volume is associated with lower haematocrit without red cell mass change or an actual reduction in red cell mass, causing relative or true anaemia, respectively. The combination of exercise and heat acclimation (which produces also hypervolaemia, but at a lesser degree than exercise) enhances hypervolaemia induced by exercise training alone. The onset of the phenomenon is extremely rapid: hypervolaemia is observed within minutes or hours of the cessation of exercise. However, 2 days are necessary to reach peak plasma volume expansion after a marathon run or longer race. The magnitude of this natural expansion ranges from 9 to 25%, corresponding to an additional 300 to 700 ml of plasma. The magnitude of this alteration depends on preceding exercise: ambient conditions, intensity and duration of exercise, body posture and frequency of the exercise bouts. The larger the reduction in plasma volume during exercise, the greater the subsequent hypervolaemia. The hydration status of the subjects before and during exercise might modify also plasma volume changes: sufficient fluid ingestion can lead to plasma volume expansion even during prolonged exercise. Fluid-regulating hormones (aldosterone, arginine vasopressin and atrial natriuretic factor) in conjunction with an elevation in plasma protein content promote hypervolaemia. However, the role and the mechanism of the increase in protein mass remain unclear and the hormonal role in the induction of chronic hypervolaemia is still an open question. Hypervolaemia can improve performance by inducing better muscle perfusion, and by increasing stroke volume and maximal cardiac output. By increasing skin blood flow, plasma volume expansion also enhances thermoregulatory responses to exercise. This leads to the important concept of optimal plasma volume and haematocrit, and performance.