Ventilatory responses to hypoxia (A) and hypercapnia (S) were measured in 127 healthy men (105 males and 22 females) by a dual control system for regulating Pa O2 and Pa CO2 simultaneously and independently of each other. The subjects were classified into groups according to sex, age, family history, and genetics (twins). Mean values for A were 138 +/- SD 93 liters/min.mmHg in group I (subjects without family history of chronic lung diseases) and 80 +/- 56 liters/min.mmHg in group II (sons of patients with chronic obstructive pulmonary disease or silicosis). The difference was highly significant (P less than 0.001). Mean S were 1.11 +/- 0.39 liters/min.mmHg-1 in group I and 1.16 +/- 0.48 liters/min.mmHg-1 in group H. The difference was not significant. Mean values for A and S in the aged males (mean age = 71.3 years) did not differ from those in 15 adult males (mean age = 29.5 years) and 60 young twins (mean age = 16.4 years). In every respect of age, sex, familial factors, and genetics, A and S were not interrelated. Mouth occlusion pressure (P0.1) was measured during air breathing and it correlated neither with A nor S. These results indicate 1) hypoxic and hypercapnic responses are not strongly related to each other, and 2) the aging process does not alter ventilatory responses to hypoxia and hypercapnia.