Patients with OSA have many episodes of increased airway resistance because of repeated collapses of upper airways during night. The aim of this work was to evaluate respiratory response during chemical stimulation without and with added inspiratory resistive load (10 cmH2O/L/sec). The studies were performed during quiet breathing with air and during hypercapnic and hypoxic rebreathing tests without and with inspiratory resistive loading in 23 obese (BMI = 34.4 +/- 4.3 kg/m2) patients with OSA and in 10 healthy subjects with similar weight (BMI = 32.4 +/- 4.3 kg/m2). The measurements of respiratory responses (ventilation, mouth occlusion pressure) were performed with the use of computerized equipment. During quiet breathing in response to added load an increase of P0.1 in controls and in OSA patients was observed. During hypercapnic stimulation the ventilatory response with additional load decreased in patients as well as in controls. The slope of mouth occlusion pressure response increased significantly in controls (from 4.40 to 6.83 cmH2O/kPa, p < 0.001) and slightly weaker in OSA patients (from 4.21 to 5.43 cmH2O/kPa, p < 0.05). Although the difference between the slopes was not significant, we found that the absolute increase of P0.1 measured at point 8 kPa of PEtCO2 during loaded breathing was significantly smaller in OSA patients in comparison to controls. (2.1 vs. 10.3 cm H2O; p < 0.001). During hypoxic stimulation occlusion pressure responses were similar in both examined groups. In conclusion we postulate that OSA patients have impaired respiratory compensation of additional inspiratory load, what was demonstrated during hypercapnic rebreathing test.