The life-span of Drosophila melanogaster males kept for their entire l
ife in atmospheres of differing O-2 concentration (21%, 30%, 40%) was
measured. The life-span decreased linearly with the increase in O-2 pr
oportion. In order to test the rate of living and the threshold theori
es of ageing, Drosophila melanogaster males were kept for 15 days in a
given atmosphere and from day 16 on in another one. The life-spans ob
served in these alternate conditions were very close to the values pre
dicted by the rate of living theory and are thus not compatible with t
he threshold theory.