Aspergillus fumigatus is a human pathogenic fungus capable of inducing a range of disease states in immunocompromised patients and has the ability of withstand attack by macrophages. Mitochondrial alternative oxidase may be important for fungal survival in macrophages, based on accumulating evidence for physiological functions of AOX in both bioenergetics and antioxidant defense. In order to evaluate the role of different ROS in the alternative oxidase activity in A. fumigatus, 2x10⁹ conidia were inoculated in YG medium and growth for 16 hours at 30ºC, in the presence of 5 mM, 30 mM and 70 mM H₂O₂, 0.5 mM menadione or 5 mM paraquat. After that, ROS donors were added again and incubated for one more hour. Conidia were harvested by centrifugation and washed three times in PBS (pH 7,4). The oxygen uptake by fungal cells was measured polarografically in Clark-type electrode in a standard medium with presence or absence of 10 mM KCN. Oxygen uptake via the normal cytochrome pathway was unaffected in the presence of stress factor. In contrast, cyanide-resistant respiration was significantly increased by the treatment with menadione and paraquat compared to control. At 0.5 mM menadione and 5 mM paraquat alternative respiration pathway comprised 70-85% of the total oxygen consumption versus 35-40% at untreated culture. Different concentrations of H₂O₂ had no effect in the AOX activity.