A photoelectric apparatus has been designed and constructed to record continuously the arteriovenous oxygen difference in animals. It operates by automatically subtracting the optical density of arterial blood at 640 millimicrons wavelength from the optical density of venous blood at the same wavelength. Calibration curves indicate that the instrument has an accuracy at least approaching that of the Van Slyke-Neill manometric analysis. Also, the electrical output of the recorder is a linear function of the arteriovenous oxygen difference. Submitted on July 9, 1956