Microclimatic conditions in the giant nests of the leaf-cutting ant Atta vollenweideri were experimentally examined to address the question whether increasing concentrations of the respiratory gases affect colony respiration. Measurements of CO2 concentrations of less than 2.8% at different depths inside mature field nests indicated good ventilation, even at 2 m depth. Passive ventilation was driven by wind and promoted by the shape of the nest. It did not influence nest temperature nor relative humidity. During rain and flooding, small colonies closed all nest openings to protect the nest from excessive water influx and damage. Measurements in a small nest indicated that, as a consequence of closure, the Co2 concentration increased rapidly. This situation was simulated in the laboratory, using a small colony of Atta sexdens as a model system. Colony respiration rates were shown to be reduced as a function of increasing CO2 concentration. Based on literature data on ant respiration, it is suggested that the respiration of the symbiotic fungus was reduced, and not that of the ants. Since the brood of leaf-cutting ants feeds exclusively on the fungus, reduced nest ventilation and its effects on respiration rates may compromise colony growth. While mature nests provide the colony with good microclimate under all weather conditions during summer, developing colonies are confronted with a tradeoff between minimizing the risk of inundation and assuring adequate gas exchange inside their nests.