Effects of acute, progressive isocapnic hypoxia on heart rate (HR) and ventilation were determined in 31 patients with chronic obstructive pulmonary disease (COPD) and in 24 normal control subjects. There was an inverse linear relationship between heart rate and SaO2 in each subject. The slope factor (delta HR/delta SaO2) obtained from the regression line was significantly higher in the patients with COPD than in the normal control subjects (0.888 +/- 0.309 SD beats/min/% fall in SaO2 versus 0.693 +/- 0.287; p less than 0.05), whereas the ventilatory response (delta VE/delta SaO2) was not significantly different between the two groups. To elucidate factors responsible for the augmented heart rate response to hypoxia in the patients with COPD, we examined the relationships of delta HR/delta SaO2 with age, physical characteristics, pulmonary function data, and arterial blood gas data in all the subjects. A weak but significant relationship was found only between delta HR/delta SaO2 and FEV1/VC, %FEV1, RV/TLC, and %RV. Because the HR response to hypoxia correlates only with parameters that reflect the grade of airway obstruction, we believe that the enhanced HR response seen in patients with COPD is a result of the disease process in the airway and tissue, although the precise mechanism was not specified in this study.