Research in many sports suggests that losing ~2% of body mass (BM) through sweating impairs athletic performance, although this has not been tested in ice hockey players. This study investigated pregame hydration, and on-ice sweat loss, fluid intake, and sodium (Na+) balance of elite male junior players during an ice hockey game. Twenty-four players (2 goalies, 7 defensemen, 15 forwards) volunteered to participate in the study (age, 18.3 ± 0.3 years; weight, 86.5 ±1.6 kg; height, 184.1 ± 1.3 cm). Players were weighed pre- and postgame, fluid and sodium intake were monitored throughout the game, and fluid and Na+ balance were determined within the time between BM measurements. Sweat Na+ loss was calculated based on sweat loss and sweat [Na+] determined from sweat-patch analysis on the same players during an intense practice. Players arrived at the rink in a euhydrated state and drank 0.6 ± 0.1 L of fluid before the game. Mean playing time for the forwards was 18:85 ± 1:15 min:s and playing time for the defense was 24:00 ± 2:46 min:s. Sweat loss was 3.2 ± 0.2 L and exceeded net fluid intake (2.1 ± 0.1 L). Mean BM loss was 1.3% ± 0.3%, with 8/24 players losing between 1.8% to 4.3% BM. Players preferred to drink water and a carbohydrate electrolyte solution before the game and during intermissions, while only water was consumed during each period. Practice mean forehead sweat [Na+] was 74 mmol·L-1. Estimated sweat Na+ losses of 3.1 ± 0.4 g (~8 g NaCl) coupled with low Na+ intake of 0.8 ± 0.2 g (~2 g NaCl) resulted in a significant Na+ deficit by the end of the game. This study demonstrated that despite abundant opportunities to hydrate during a hockey game, one-third of the players did not drink enough fluid to prevent sweat losses of 2% BM or higher. Losing 2% BM has been associated with decreases in athletic performance.