THE determination of cardiac output by the dye dilution techniques requires the taking of serial arterial blood samples at short regular intervals. Direct puncture of the brachial artery of cattle1 enables such samples to be collected. T.1824 (Evans blue) was injected into the jugular vein of the subjects at a dose of approximately 0.40 mgm./kgm. body-weight and arterial blood samples collected at 1-sec. intervals for a period of 30 sec. Adopting the procedure described by Hamilton et al. 2 the average concentration of the dye in the arterial blood samples was found, together with the time required for one circulation of the dye and the cardiac output calculated from the formula: