We studied the effect of acute progressive hypoxia on heart rate in conscious healthy subjects. The PCO2 was held constant (+/- 1.5 mmHg) midway between the resting end-tidal and mixed-venous levels. Hypoxia was induced by having the subject rebreathe from a small bag so that the PO2 fell at a rate related to the subject's oxygen consumption. Arterial oxygen saturation (Sao2) was measured continuously during the procedure with an ear oximeter. We found that heart rate (HR) was best fitted to an inverse linear relation to arterial oxygen saturation and a power relation to PO2. The range of deltaHR/deltaSao2 was 0.62-1.46 beats/min per 1% fall in Sao2 (mean +/- SE = 0.98 +/- 0.06). There was no relationship between heart rate and ventilatory responses to hypoxia.