Multiple measurements of left ventricular volumes, ejection fractions, and circulatory pressures were made under conditions of varied outflow resistance in five intact anesthetized dogs by using a biplane videoangiographic recording system which displays simultaneously two roentgen images on the same television screen, records the images on video tape, and requires low radiation levels and small amounts of contrast medium. Aortic pressure was controlled by intra-aortic infusion of angiotensin or acetylcholine. With spontaneous sinus rhythm and normal aortic pressure, end-diastolic volume averaged 35 ml (2.3 ml/kg), and ejection fraction averaged 47% of end-diastolic volume. When aortic pressure was increased, average end-diastolic volumes increased to 53 ml and ejection fractions decreased to 29%. Conversely, when aortic pressure was decreased, average ejection fraction increased to 73%, although changes in stroke volume were usually not large under these conditions. Concomitant large changes in the shape of the left ventricular cavity were also observed. Similar changes were observed when heart rate was maintained constant by electrical pacing of the atria and ventricles with a constant atrial-ventricular stimulus interval. The variability in left ventricular volume and ejection fraction observed in the same animal during different circulatory states may account in part for the differences in values for these variables reported by different investigators.