The response of ventilation and of heart rate to hypoxia and hypercapnia was determined in eight young normal men age 22-30 yr and eight elderly men age 64-73. The elderly men were selected and carefully screened to eliminate the possibility of cardiopulmonary disease. All the subjects were born at low altitude and had no significant prior exposure to hypoxia. The ventilatory response to hypoxia was measured as the exponential slope constant. k, of regression lines relating the logarithm of incremental ventilation to PAo(2) during isocapnic progressive hypoxia. The heart rate response to hypoxia was measured as the percentage change in heart rate between PAo(2) = 100 and PAo(2) = 40 mm Hg. The ventilatory response to hypercapnia was measured as the slope of regression lines relating ventilation to PAco(2) during rebreathing with PAo(2) > 200 mm Hg. The heart rate response to hypercapnia was measured as the percentage change in heart rate between control values at the start of the rebreathing test and PACO(2) = 55 mm Hg. The ventilatory and heart rate responses to both hypoxia and hypercapnia were significantly decreased in the elderly men as compared to the young men. Hypoxic ventilatory drive was decreased by 51+/-6% (mean +/-SEM: P < 0.001) and hypercapnic drive by 41+/-7% (P < 0.025). The percentage change in heart rate produced by hypoxia was 34+/-5% (mean +/-SEM) in the young normals and 12+/-2% in the old normals (P < 0.005). Similar figures for heart rate in response to hypercapnia were 15+/-3% and -1+/-1% for the young and old normal groups (P < 0.001). We conclude that ventilatory and heart rate responses to hypoxia and hypercapnia diminish with age. These alterations in both ventilatory and circulatory controls could make older individuals more vulnerable to hypoxic disease states.