Body hydration decreases significantly during hypokinesia (HK) (diminished movement), but little is known about the effect of fluid and salt supplements (FSS) on body hydration during HK. The aim of this study was to measure the effect of FSS on body hydration during HK. Studies were done during 30 days pre HK period and 364 days HK period. Thirty male athletes aged 24.5 +/- 6.6 yr were chosen as subjects. They were equally divided into three groups: unsupplemented ambulatory control subjects (UACS), unsupplemented hypokinetic subjects (UHKS) and supplemented hypokinetic subjects (SHKS). Hypokinetic subjects were limited to an average walking distance of 0.7 km day-1. The SHKS group took daily 30 ml of water/kg body weight and 0.1 g of sodium chloride (NaCl)/kg body weight. Control subjects experienced no changes in their professional training and routine daily activities. Plasma volume (PV), urinary and plasma sodium (Na) and potassium (K), plasma osmolality, plasma protein, whole blood hemoglobin (Hb) and hematocrit (Hct), plasma renin activity (PRA) plasma aldosterone (PA) levels, physical characteristics, food and fluid intakes were measured. Plasma osmolality, plasma protein, urinary and plasma Na and K, whole blood Hct and Hb, PRA and PA levels decreased significantly (p < or = 0.01), while PV and body weight increased significantly (p < or = 0.01) in the SHKS group when compared with the UHKS group and did not change when compared with the UACS group. Plasma osmolality, plasma protein, urinary and plasma Na and K, PRA and PA, whole blood Hb and Hct levels increased significantly (p < or = 0.01), while PV body weight, food and fluid intakes decreased significantly (p < or = 0.01) in UHKS group when compared with the SHKS and UACS groups. The measured parameters did not change in the UACS group when compared with their baseline control values. It was shown that during HK body hydration decreased significantly, while during HK and FSS body hydration increased significantly. It was concluded that daily intake of FSS prevents the decrease of PV and blunts the increase of activity of the PRA and PA during prolonged HK.