Six adult males of similar body composition and aerobic capacity were tested to study the effects of changes in plasma volume (PV), osmolality (OSM) and sodium (Na+) on core temperature (Tc) under three exercise-thermoregulatory stress conditions. The protocol consisted of 120 min of upright stationary cycling at 50% V02max under neutral (24° C, 50% RH) - euhydrated (NE), hot (35°C, 50% RH) - euhydrated (HE), and hot-hypohydrated (HH) environmental conditions. Venous blood samples were obtained at -30 min, 0 min and at 15 min intervals through a 30 min recovery and were analyzed for blood hematocrit and hemoglobin, and for plasma osmolality and sodium. Hematocrit and hemoglobin were used to calculate relative changes in plasma volume. Tc showed qualitatively similar linear increases in the first 45 min of each trial. At 60 min, Tc in the NE trial plateaued at 37.9°C. In the HE trial, Tc continued to show a slight further increase after 45 min while in NE it became significantly (p<0.05) lower at 45 min as compared to HE and HH; at 60 min of exercise, the core temperature of all three trials differed significantly (p<0.05), with HH being the highest (38.3°C). Percent change in plasma volume was not different between trials, but did show the greatest decrease in all trials from O to 15 min of the exercise phase with at least -4.3%. Osmolality was significantly different (p<0.05) between the NE (X = 283.3 m0smol/kg) and the HH (X = 292.5 m0smo1/kg). Plasma sodium was significantly (p<0.05) higher for all intervals of HH (X = 137.9 meq/L) as compared to the NE (X = 135.1 meq/L) and HE (X = 134.8 meq/L). These data suggest that core temperature (Tc) increase in moderate intensity endurance exercise is less related to a decreased circulating plasma volume, but is more strongly associated with rising osmolality, specifically the increase in the Na+ electrolyte, which occur with progressive hypohydration.