The functional properties of haemolysates from rodents and bats and their related parameters such as haemoglobin components and concentrations, haematocrits, erythrocyte number and haematimetric index were studied. Burrowing Microtus (Pitymys) duodecimcostatus has greater oxygen affinity than non-burrowing Apodemus sylvaticus. This high affinity is the most common adjuster of oxygen transportation in burrowing mammals as a response to the hypoxic medium in which they live. The higher haemoglobin affinity in the vole with respect to A. sylvaticus may compensate for the more intense Bohr effect found in M. (P.) duodecimcostatus, and thus facilitate oxygen liberation in the tissues. Bat haemoglobin O₂ affinity is higher than in similarly-sized non-flying normothermic mammals. The Bohr effect in bats is slightly lower than those reported for small non-flying mammals. The levels of 2,3-DPG high in red blood cells of active bats decrease when the bats are hibernating. Thus changes in haemoglobin O₂ affinity are more probably due to changes in 2,3-DPG concentrations than to alterations of body temperature. Multiple haemoglobin components have been demonstrated by polyacrylamide gel electrophoresis in these species. This high number of components may be related to the habitat.