Effects of fat adaptation and NaHC0₃ supplementation on acid-base homeostasis were quantitated during repeated sprint exercise in horses. Contribution of strong ions ([SID]), Pco₂, and weak electrolytes ([A_tot]) to changes in plasma [H⁺] and the role of erythrocytes in acid-base balance were examined at rest and during exercise. Effects on plasma glucose and blood lactate [Lac⁻] concentrations due to sample handling were also assessed. During exercise, blood [Lac⁻] was higher when horses received NaHC0₃ compared to water prior to exercise (P=0.0024), and in fat adapted horses compared to horses fed a control diet (P=0.0240). Blood [Lac-] was higher in fat adapted horses given NaHC0₃ compared to other diet/treatment combinations (P=0.0276). Plasma [SID] was higher during exercise when horses were given NaHC0₃ compared to plain water (P=0.0054), which contributed to decreasing [H⁺] and increasing [HC0₃⁻] during exercise (P=0.0001). Plasma Pco₂ contributed less to increasing plasma [H⁺] during exercise in fat-adapted horses compared to horses fed the control diet (P=0.0282). Intraerythrocyte [SID] decreased (P=0.0160) and [Atod increased (P=0.0002) which contributed to increasing [H⁺] within the cell (P=0.0228). / Ph. D.