Whether genetic influence on chemical and behavioral control of breathing is still present in adulthood was examined in 28 pairs of monozygotic (mean age, 40 +/- SD 9.9 yr) and 10 dizygotic (35 +/- 9.3 yr) twins. Mean values for hypoxic and hypercapnic ventilatory responses, threshold for perceiving added inspiratory resistance, and respiratory patterns were not different between monozygotic and dizygotic twins. However, within-pair variance ratios (those in monozygotic twins being denominators) for hypoxic response (4.08, p less than 0.005), hypercapnic response (4.89, p less than 0.005), respiratory frequency during air breathing (3.96, p less than 0.005), inspiratory time during air breathing (5.47, p less than 0.005), and inspiratory time during hypoxia (9.08, p less than 0.005) were significantly larger than 1. Within-pair variances for threshold for resistive load perception and respiratory pattern during hypercapnia were equivalent between the 2 groups. These results indicate that ventilatory responses to hypoxia and hypercapnia and respiratory patterns while breathing air include genetically determined factors, whereas perception threshold for added resistance and respiratory patterns during hypercapnia are influenced predominantly by environmental force in adulthood.