Left ventricular volumes were measured by cineangiocardiography in 56 sessions on 25 reindeer, together with determinations of arterial pressure and blood oxygen saturation. The heart rate with the animal kept lying on the sternum at rest was 50 b/min, the aortic blood oxygen saturation 94 to 98% and the aortic blood pressure 153/130/115 mm Hg. The left ventricular end-diastolic volume was 3.4 ml/kg of b.w., stroke volume 2.6 ml/kg, cardiac output 133 ml/kg . min, and ejection fraction 76%. If the animal was placed on its side the aortic blood oxygen saturation decreased by 2 to 3 per cent, end-diastolic volume rose 10%, stroke volume 25%, cardiac output nearly 30% and ejection fraction 14%. The heart rate and cardiac output increased in hypoxemia, but the end-diastolic and stroke volumes simultaneously decreased. If the aortic blood pressure rose during hypoxia, the end-diastolic and stroke volumes increased, while the ejection fraction first increased and then decreased again. In normoxia a rise in aortic blood pressure was associated with decreased end-diastolic and stroke volumes and ejection fraction. An increased heart rate at rest was followed by increased cardiac output, but decreased end-diastolic and stroke volumes and ejection fraction.