The data presented in this article illustrate the dynamics of the affective sphere of the mental processes associated with the higher nervous activity and the characteristics of the neuroendocrine system (including the vegetative index of Kerdo, cortisol and serotonin levels) in the participants of the marathon swimming competition covering the distance of 101.4 km. We investigated the relationship between the levels of the above hormones and the functional activity of the vegetative nervous system. Aims — the objectives of the present study were (1) to determine the dynamics of the changes in the psychophysiological characteristics of the participants in the marathon swimming competition during which the world record for the covering of the 101,4 km distance in the ice water was set and (2) to follow up the development of the stress response at different levels of the life support system. Materials and methods. A total of 22 participants in the competition were available for the examination designed to evaluate their psychological status and hemodynamic parameters as well as to determine the cortisol and serotonin levels in the blood sera of these subjects. Results. The data obtained in this study give evidence that before the start and during the marathon distance swimming, only positive emotions prevailed among the participants whereas the setting of the world record was followed by the progressive development of the state of indifference. The dynamics of the Kerdo vegetative index suggests the balance between the functioning of the sympathetic and parasympathetic parts of the autonomous nervous system at the start of the competition and the prevalence of sympathetic activity in the autonomic nervous system at its finish. The dynamics of the level of cortisol in the serum indicates the presence of a stress. Conclusions. Despite the lack of the proper sports training, the participants in the marathon distance swimming competition proved capable of overcoming a variety of physical and psychological workloads which made it possible to set the world record. On the one hand, this confirms that the experience with winter swimming gained during a few years is sufficient to enable the swimmers to participate in such events. On the other hand, the results of the study give the idea of the «price» to be paid by the physiological and psychological components of the life support system of the participants in the marathon distance swimming competitions seeking to set a world record.