Irradiation shortened life in triclads by an amount which depended, though not linearly, on dose. Young (3 months) and old (24 months) individuals were more susceptible than middle-aged (12 months) individuals. The greater sensitivity of young triclads was ascribed to an injurious effect of irradiation whereas the greater sensitivity of older triclads was ascribed to the ageing effect. Degrowth, followed by regrowth, but not regrowth alone, retarded the ageing effect. From this it was deduced that the active agent of rejuvenation was regrowth and not degrowth.