Cot death could be the result of chance coincidence of an adventitious challenge and a baby ill-equipped to meet it. This work was designed to test the hypothesis that there is a subpopulation of babies who lack the appropriate responses to hypoxia and hypercapnia which would enable them to overcome the effects of, for instance, nasal obstruction. The responses of 630 babies to air mixtures which induced significant changes in ventilation in the overwhelming majority, were recorded in a short protocol in which both the addition and withdrawal of hypercapnia and of hypoxia were effected. The results of each test were placed in one of five categories; in 13.6% there was no response to hypoxia, and in 2% the ventilation fell in hypoxia to a significant degree. The study confirms the existence of a subgroup of normal babies with little defence to the respiratory loading of mild upper respiratory tract infections.