The present study was conducted to investigate the in vivo effects of an intrahepatic infusion of deionized water during exercise in rats. Adrenodemedullated male Sprague-Dawley rats were continuously infused for 30 min either at rest or during treadmill exercise (26 m/min, 0% grade). Rats were randomly assigned to one of three infusion conditions (52 µl/min) with either deionized water (PW) or saline (PS; NaCl; 0.9%) via the hepatic portal vein or deionized water through the jugular vein (JW). The exercise period caused a significant (P < 0.05) decrease in liver glycogen and relative liver water content and peripheral and portal blood glucose and insulin while increasing peripheral and portal glucagon and K⁺ plasma concentrations. These responses, with the exception of K⁺, were not influenced by the different types of infusions. The increase in K⁺ during exercise was significantly (P < 0.05) higher in JW rats than in the PW and PS groups. Both the infusion and exercise protocols did not significantly alter the liver weight-to-body weight ratio, plasma osmolality, free fatty acids, -hydroxybutyrate, Na⁺, Cl, vasopressin, and catecholamine concentrations. It is concluded that an hepatic portal infusion of deionized water does not specifically alter the metabolic and hormonal responses to exercise in rats.