The aim of this study was to evaluate the effect of a daily intake of fluid and salt supplementation in increasing the capacity of endurance‐trained male volunteers for maximum physical exercise after an exposure to 364 days of hypokinesia (decreased number of steps/day) and during a posthypokinetic period of 35 days. The studies were performed after exposure to hypokinesia of 364 days and during the posthypokinetic period of 35 days on longdistance runners who had an average maximal oxygen uptake (VO2max) of 65 ml/kg‐min and ranged from 19 to 24 years of age. Prior to their exposure to hypokinesia, all volunteers completed an average of 14.9 km/day. All volunteers were divided into three equal groups: Group 1 underwent a normal ambulatory life (control subjects), group 2 were placed under continuous restriction of their motor activity being allowed only an average of 2.7 km/ day (hypokinetic subjects), and group 3 also kept continuously to an average of 2.7 km/day but consumed a daily additional amount of 26 ml water/kg body weight and 0.16 g sodium chloride/kg body weight (hyperhydrated subjects). During the experimental period, heart rate, stroke volume, blood pressure, plasma volume, hemoglobin, hematocrit, plasma osmolality, plasma protein, sodium, and potassium concentrations in plasma and water balance were regularly measured. A graded physical exercise test was also performed regularly. During the posthypokinetic period, the capacity of hyperhydrated volunteers to perform maximum physical exercise increased significantly; the group mean heart rate decreased, while stroke volume and blood pressure of the subjects increased. Fluid retention was also much higher in the hyperhydrated hypokinetic than in the hypokinetic‐only volunteers. Plasma volume increased, and hematocrit, hemoglobin, plasma osmolality, plasma protein content, and sodium and potassium concentration in plasma decreased in the hyperhydrated hypokinetic subjects. In the hypokinetic‐only subjects the reverse change was observed. It was concluded that the increased physical capacity of the hyperhydrated subjects for maximum physical exercise after exposure to hypokinesia depends upon the level of body hydration during prolonged restriction of muscular activity.