Purpose. pH- blood level is critical for life. Alterations in acid base status which may lead to incorrect clinical decisions will be discussed. There is a narrow range of pH values that the human body and its enzyme-supported system operates within. Changes in venous pH during maximal exercise in heart failure (HF) patients is not well studied. The purpose of the present study was to measure the time-related changes in venous pH during maximal exercise in untrained healthy volunteers and heart failure patients with III NYHA class. Methods. 17 untrained healthy volunteers, mean age 26+/-7 years, 15 men, BMI 24+/-4; and 38 heart failure (HF) patients with III NYHA class, mean age 48+/-11 years, 32 men, BMI 27+/-2 were evaluated during progressive treadmill exercise. The ventilatory variables were recorded breath-to-breath (Oxycon Pro, Jaeger, Germany). The cubital venous catheter was installed in all subjects before exercise test. Blood samples were taken at baseline and at 1-minute intervals during test. PH, lactate and HCO3- concentration were estimated using analyzer i-STAT, cartridge CG4 (Abbot, USA). Results. Oxygen consumption at peak exercise in HF patients was lower than that in healthy volunteers: 13.4+/-23.7 and 38+/-5.3 ml/min/kg, respectively, p < 0.05. Venous pH mean values at baseline were 7.39+/-0.03 and 7.38+/-0.04 in untrained healthy volunteers and HF patients, respectively, p > 0.05. Venous pH mean values at peak exercise were 7.17+/-0.05 and 7.36+/-0.05 in untrained healthy volunteers and HF patients, respectively, p < 0.05. Conclusions. The pH-range during maximal exercise in heart failure patients with III class is narrower than that in untrained healthy volunteers.