Studies in human subjects have shown considerable variation in the ventilatory response to hypoxia among individuals. The potential influence of genetic factors was initially suggested by the finding of clusters of low responses in families of patients with unexplained hypoventilation and endurance athletes. Further evidence of a genetic effect was evident in studies which found greater similarity of hypoxic ventilatory response in monozygotic than in dizygotic twins. The apparently selective effect on the hypoxic versus the hypercapnic response suggests that the genetic influence is related to peripheral chemosensitivity. Studies in cats and rats also point to a genetic effect on the carotid body.