Changes of beta-endorphin level in blood during prolonged exercise

In 49 healthy subjects during 2 h exercise on bicycle ergometer four variants of changes of beta-endorphin level were observed: 1. an increase during first 30 min followed by a decreased below initial values; 2. biphasic increase (peak values at the 30th and 120th min); 3. increase only during the 2nd h of exercise; 4. decrease during the whole period of exercise. For trained subjects the most common was the biphasic increase, for untrained ones--the overall decrease.