The prolongation of the life span of human fibroblasts in vitro by hydrocortisone seems to be associated with its stimulatory action on ribosome synthesis. The sustaining effect of the hormone on the latter remains, after withdrawal of hydrocortisone, for a time which is dependent on the age of the cells. When hydrocortisone is removed from cultures that have grown beyond the life span of the controls, the cells die within two passages. Hence, the events leading to the growth decline took place in spite of the presence of the hormone which does not seem to prevent the aging process but rather to delay it. The results could be compatible with either the error hypothesis or the theory which explains aging by an increased binding of histones to DNA with the subsequent blocking of genetic information.