Oxygen dissociation curves plotted directly on whole blood during oxygenation using a Radiometer DCA 1 gave P50 values of 39.2 +/- 2.2 mm Hg and 51.3 +/- 6.2 mm Hg for pigeon and chicken blood, respectively, at 41 degrees C, pH 7.5 and PCO2 29 mm Hg. These values agree well with earlier results obtained by a variety of methods; however, they do conflict with recent findings of a much higher oxygen affinity in avian blood which were obtained using a different plotting method. It was shown that with the method used in this paper the high metabolic rate of avian blood has little effect on the plotted curve at saturations below 50%. At higher saturations O2 consumption of red cells affects a progressive shift of the curve to the left so that to obtain accurate curves at high saturation it is necessary to correct the plotted curves for the oxygen utilization.