An arterial or venous blood gas is a clinical tool for determining pulmonary and metabolic status. Arterial Blood Gas (ABG) or Venous Blood Gas (VBG) methods provide a direct measurement of partial blood pressures of carbon dioxide (PaCO2) and oxygen (PaO2), hydrogen ion activity (pH), total hemoglobin (Hbtotal), oxyhemoglobin saturation (HbO2) and bicarbonate ion concentration (HCO3
-). Most blood tests are done on a sample of blood taken from a vein due to: 1-Collecting blood from an artery is more painful than collecting it from a vein because the arteries are deeper and have more nerves, 2-Artery may be inaccessible due to periarterial tissues (overlying muscle, connective tissue, or fat). In ruminants, feeding diets high in grain and other highly fermentable carbohydrates increases the risk of ruminal and blood acidosis. Although ruminal pH varies considerably within a day, cows possess a highly developed system to maintain ruminal pH within a physiological range. However, if the acid production from fermentation is more than the system can buffer, ruminal pH compensation fails and it may drop drastically (Marie Krause & Oetzel, 2005). The importance of arterial or venous blood gas measurements in the diagnosis of ruminal acidosis is prevented some health problems such as ruminal parakeratosis, erosion and ulceration of the ruminal epithelium (Garry, 2002), laminitis, sole abscesses and sole ulcer (Nocek, 1997). The objective of the present experiment was to investigate the effect of diets providing different concentrate: lucerne hay ratios on venous blood gas in Holstein steers.