Ventilatory, tidal volume and frequency responses to progressive isocapnic hypoxia have been measured in twenty-nine healthy subjects by a rebreathing technique. 2. A strong correlation was found between ventilatory response to hypoxia (deltaVI/DELTASaO2) and frequency response to hypoxia (deltaf/deltaSaO2) (r=0-82, P less than 0-001). There was a lesser correlation between deltaV1/deltaSaO2 and tidal volume response (deltaVT/deltaSaO2) (r=0-50, P less than 0-01). These findings suggest that the wide range of ventilatory response to hypoxia among subjects is mainly determined by differences in frequency response and contrast with previous findings in studies of the response to progressive hypercapnia. 3. The breathing pattern during progressive hypoxia and hypercapnia was compared in ten subjects. Ventilation/tidal volume plots were constructed and patterns of response were further analysed in terms of inspiratory duration (TI), expiratory duration (TE) and mean inspiratory flow rate (VI). 4. Increments in ventilation during hypoxia were achieved with a greater respiratory frequency and a smaller tidal volume than during hypercapnia in eight of the ten subjects studied. In two subjects no difference in breathing pattern during hypoxia and hypercapnia was observed. 5. Changes in respiratory frequency during progressive hypoxia were achieved in all subjects by a progressive shortening of TI and TE. By contrast, TI remained constant during hypercapnia until VT had increased to 3-5 times the eupnoeic value; during hypercapnia the increase in frequency was achieved mainly by a progressive shortening of TE. 6. It is concluded that different mechanisms may be involved in altering respiratory frequency when ventilation is driven progressively by these different chemical stimuli.
