The oxygen dissociation curve of partially NO‐liganded hemoglobin of human erythrocytes is measured. As the percentage of NO ligation increases, the affinity of nonliganded hemoglobin for oxygen increases and the heme‐heme interaction decreases; furthermore, methemoglobin is formed. Therefore, NO affects the oxygen transport function of hemoglobin, decreasing the oxygen supply to peripheral tissues, because of (1) simple diminution of the available hemoglobin by the tightly bound NO, (2) the high affinity of hemoglobin for oxygen, and (3) the inevitable formation of methemoglobin.