We have studied the hormonal and vegetative responses to reactions of exercise in 60 healthy subjects (10 young and 50 old) and in 70 patients with ischemic heart disease (IHD) aged 60-74 years. Plasma levels of cortisol, ACTH, met-enkephalins, leu-enkephalins and beta-endorphins were measured by using radioimmunological methods. The state of autonomic control was determined according to spectral analysis of cardiac rhythm. Compared to the young healthy subjects, the older healthy group had an earlier activation of the ACTH-cortisol system and a more pronounced increase in the sympathetic nervous system during sub-maximal exercise. This could be the reflection of a decreased resistance to stress in older age. The elderly IHD patients showed two types of hormonal reactions to exercise: a rise in the cortisol level and a fall in the met-enkephalins (Group 1), or a fall in the cortisol concentration combined with a rise of met-enkephalins (Group 2). In Group I patients a lower exercise capacity and a less economical reaction of the hemodynamics to a workload of 25 watts was found. An excessive activation of the ACTH-cortisol system and a reduced level of blood met-enkephalins in the elderly IHD patients during exercise are thought to be the unfavourable factors promoting a decrease in tolerance to physical load and an impairment of adaptive capacities of the cardiovascular system.