RESUMO
Rabbits were kept for the whole of their natural life-span (6--9 years) after exposures of 440--1410 rad of gamma-rays or 180--550 rad fission neutrons. Irradiated rabbits did not live as long as unirradiated controls but the difference was statistically significant only after the higher doses of neutrons. Earlier deaths were explained by increased incidences of a wide variety of tumours. The increases were statistically significant for basal-cell tumours of the skin, fibroasarcomas, osteosarcomas and Sertoli-cell tumours of the testis. Other phenomena assoicated with natural ageing were unchanged or decreased in irradiated rabbits. The r.b.e. of the neutrons was assessed as 3--3.5 for tumour induction and 3x5--4x0 for longevity. Comparisons with previous data on acute and subacute effects suggest that the r.b.e. increased linearly with the logarithm of time for 5--2000 days after irradiation.