INTRODUCTION 1. Since the 1958 and the 1962 reports of UNSCEAR [U1, U2] the Committee has not reviewed the evidence accumulating in the field of non-neoplastic longterm effects of whole-body irradiation. The scope of the present document is to consider the data available in order to ascertain: - the existence and extent of life-span shortening in irradiated animals and man as a function of the main physical and biological variables known to influence this effect of radiation; - whether and to what extent and within what ranges of the above-mentioned variables this effect might be attributed by careful pathological analysis to real non-tumorous conditions or, on the contrary, to specific neoplastic diseases; - whether and within what range of doses a non-specific radiation effect may be traced and quantified; - whether such a non~specific shortening of life might be considered akin to the normal biological aging process. Although some of the above problems may appear too ambitious in the light of the present biological and radiobiological knowledge, the Committee believes that a review of the field with such objectives in mind may be of value at least as a selective collation of the existing information. A. 2. HISTORICAL BACKGROUND Radiation-induced life-span shortening was described first in the rat by Russ and Scott [R1] and in the mouse by Henshaw [H1]. They reported that irradiated animals had a shorter duration of life, looked older and appeared to age more rapidly than their non-irradiated controls. These and other observations led quite naturally to the establishment of a link between the life-shortening action of radiation and natural senescence. In 1952 Brues and Sacher [B1] discussed the problem of radiation-induced long-term radiation lethality in a rather articulate way. They recognized, for example, the notion that single acute exposures to radiation tended to displace upward the Gompertz age-mortality function, while chronic exposure throughout life