This thesis investigated the main acid-base constituents in equine plasma that exhibit daily variation. A major focus was to determine total carbon dioxide concentration ([TCO2]). Jugular venous blood was sampled every 1 to 2-h over 25-h from 10 resting and either fasted or fed Standardbreds on a 3-wk conditioning program and racehorse diet. The independent variables, strong ion difference ([SID]), total concentration of weak acids and bases ([Atot]), and partial pressure of carbon dioxide (PCO2), were assessed to determine factors affecting hydrogen ion concentration ([H +]) and calculated [TCO2] using the physicochemical approach to acid-base balance. Variations were found in [glucose], haematocrit (Hct), plasma proteins ([PP]), chloride ([Cl-]), bicarbonate ([HCO3-]), [H+], [SID], [A tot] and PCO2; all of which appeared to be due to effects of feeding or dehydration. Mean [TCO2] varied up to 10 mmol/L throughout the day and 70% of the horses had a calculated measurement above 36.0 mmol/L.