Genetic selection on erythrocyte 2,3-diphosphoglycerate (DPG) levels has produced two true breeding rat lines from a Long-Evans hooded population, one showing high DPG levels and the other low DPG levels. The differences in DPG are due to a single gene (Dpg), two allele (D and d) system in which D is dominant to d. The biochemical mechanism for the DPG differences involves the enzyme, phosphofructokinase (PFK). Dpg is closely linked to Hbb, the locus for IIIIβ globin, which is also linked to the albinism (c) locus. The Long-Evans hooded commercial population, which has been kept outbred for about 200 generations, shows severe linkage disequilibrium involving alleles at Dpg and Hbb. The P50 of the oxygen dissociation curve of the high DPG animals, as well as the mixed venous Po2, is 6 torr higher than that of the low DPG animals. High DPG animals have a lower serum cholesterol, a larger litter size, and a faster rate of learning than do low DPG animals. However, because these two lines are not inbred, the data show considerable variation, and the interpretation of the cause of the differences is not clear-cut. An inbred line (LOU/Mn) with low DPG levels has been studied. The major difference in DPG levels between LOU/Mn and the high DPG Long-Evans hooded line is caused by allelic differences at a single locus. It has been shown that this DPG level-determining locus is linked to Hbb and to c. This linkage makes it almost certain that the determination of low DPG in LOU/Mn animals is due to a low DPG allele at the Dpg locus. Other data make it reasonably certain that this allele in LOU/Mn is d, the same allele that is present in the low DPG Long-Evans hooded line. With this information, a LOU/Mn congenic line segregating for D and d can be developed. Such a congenic line will make it possible to study in a very “clean” genetic system the effect of oxygenation differences on the biology of the animals. Because of the importance of oxygenation, variation in oxygen affinity probably has multiple effects on the biology of mammalian organisms. This genetic approach to producing variation in oxygen affinity has great potential for identifying and studying those areas of the biology of an organism which are sensitive to variation in oxygenation.