Under normal conditions a linear relationship exists between venous and arterial pH values. However, with severe metabolic and hemodynamic stress this correlation has not been established. To evaluate this relationship, nine dogs were subjected to 60 minutes of hemorrhagic shock at a mean arterial pressure of 30 mm Hg and were then resuscitated. Seven dogs were rendered hypothermic by surface cooling. Serial arterial and mixed venous samples for blood gas analysis were obtained from the femoral artery, superior vena cava (SVC), right atrium (RA), and pulmonary artery (PA). There were no statistical differences between mixed venous pH values obtained from SVC, RA, and PA during any study time. The differences between arterial and mixed venous pH (AVpH) had a significant linear relationship (r > 0.90, P < 0.01). The AVpH during hypothermia was +.03 to + .04 pH units. The greatest AVpH occurred during shock, and ranged from + .08 to + .20 pH units. During recovery AVpH was intermediate at + .04 to + .13. It appears that during hemorrhagic shock and moderate hypothermia, a significant relationship persists between mixed venous and arterial pH. The mixed venous sample can be obtained from the superior vena cava, right atrium, or pulmonary artery.