We examined the blood of four dasyurid marsupials, all small, active insectivores and/or carnivores, to establish if there were any specializations in oxygen transport. All four species had hemoglobins with a low affinity for oxygen. This was demonstrated by high $P_{50}$'s (the partialpressure of oxygen [PO₂] at which the hemoglobin is half-saturated with oxygen) of the whole blood (means of 38.3-59.8 mmHg) and of red cells suspended in a physiological buffer (means of 41.6-49.0 mmHg) at a CO₂ tension of 43 mmHg. There was a strong correlation betuween increase in body mass over three orders of magnitude and increase in the oxygen affinity of the whole blood. The effects of changing CO₂ tensions on the oxygen affinity of whole blood were measured as △log $P_{50}/{\Delta}log$ PCO₂; mean values ranged from 0.25 to 0.373. Values for △log $P_{50}/{\Delta}pH$ wuere measured for red-cell suspensions; means values ranged from -0.40 to -0.481. The introduction of 2,3-diphosphoglycerate significantly decreased th...