In this contribution we studied the developmental phenotypic plasticity of respiratory structures involved in the exchange of gases in an altricial rodent, the leaf-eared mouse {Phyllotis darwini). We studied morphological pulmonary parameters of warm (30 °C) and cold acclimated animals 15 °C) at different developmental stages. We found that acclimation treatments did not affect neither lung volume, the alveolo-capillary barrier thickness nor the respiratory surface density. In consequence the oxygen diffusion capacity was not affected. Although Phyllotis darwini exhibited structural respiratory changes expected by its ontogenetic development, these structures were not affected by thermal acclimation during the post-natal development.