The parallel response of sweat rate and urine production to changes in plasma osmolality and volume support a role for arginine vasopressin (AVP) as the main endocrine regulator of both excretions. Ten moderately trained runners completed both a maximal test to exhaustion and a steady-state run on a motorized treadmill, one week apart. Sweat, urine and serum sodium concentrations ([Na+]) were evaluated in association with the plasma concentrations of cytokines, neurohypophyseal and natriuretic peptides, and adrenal steroid hormones. When data from both the high intensity and steady state runs were combined, significant linear correlations were noted between: sweat [Na+] versus post-exercise urine [Na+] (r = 0.80; p<0.001), post-exercise serum [Na+] versus both post-exercise urine [Na+] (r = 0.56; p<0.05) and sweat [Na+] (r = 0.64; p<0.01) and post-exercise urine [Na+] versus post-exercise plasma arginine vasopressin concentration ([AVP]P) (r = 0.48; p<0.05). A significant positive correlation was noted between post-exercise [AVP]P and sweat [Na+] during the steady-state condition only (r = 0.66; p<0.05). These correlations suggest that changes in serum [Na+] during exercise may evoke corresponding changes in sweat and urine [Na+], which are likely regulated coordinately by changes in [AVP]P to preserve body fluid homeostasis.