This study aimed to determine the effects of different dietary protein levels on blood variables, plasma volume, and maximal oxygen uptake (VO2max) in sled dogs undergoing rigorous training. 32 Alaskan Sled Dogs were randomly assigned to 1 of 4 groups each fed isocaloric diets containing varying protein percentages (18%, 23%, 29% or 35%) for 1 month before and during a 12 week training period. Body weight, protein and energy intake, plasma volume, PCV, haemoglobin concentration, and serum biochemical variables were measured at 0, 8, and 12 weeks. Results showed that dogs fed 18% protein had greater soft tissue injuries than the other groups and, at 12 weeks had lower sodium and haemoglobin levels than dogs fed 29 or 35% protein. At 12 weeks, dogs fed 35% protein had higher plasma volume which may provide performance advantage.