lating an induction period in life which may be shortened by various treatments and an ensuring period of development, which would be relatively constant in duration. In the male Sprague-Dawley rat the period of development would begin in the age range of 10 to 15 months. 280. In a subsequent paper (J4] the relevant data for tumour induction were re- ported. There was an excess proportion of animals with one or more palpable tu- mours (compared to the control groups) after exposure at all except the oldest age (21 months), in spite of a significant life-shortening only after exposure at the three younger ages. The percentage of animals with palpable tumours was higher in all groups (even for the group exposed at 21 months) in comparison with the control tumour-bearing animals. It should be recalled that this strain of rats has normally a very high incidence of radiation-induced tumours, particularly of the skin and skin adnexa, which might have altered to an unknown extent any more precise estimates of life-shortening. D. 281. CONCLUSIONS From all the data reviewed above it may be concluded that among the bio- logical variables determining the life-shortening response to irradiation there appears to be sufficient data for discussions on the genetic constitution and on the influence of sex and of age at irradiation. 282. It is easy to understand that different species might show a different response in relation to the longevity of each species and to its specific physiologic and pathologic characters. Much less easy is to trace a common para- meter or a set of parameters on which one could evaluate the sensitivity of a species in order to construct a susceptibility scale which might facilitate inter-species comparisons or extrapolations. The analysis of survival para- meters according to the actuarial model of Gompertz; the calculation of semiempirical parameters like the excess death-rate divided by the exposure rate in chronic irradiation experiments; the evaluation of the life-shortening effect normalized as a percentage of the control value and as a function of the acute LD 50/30? the hypothesis of a common life-shortening effect per unit dose normalized according to the respective life-span of the species compared; were all criteria proposed in order to achieve the scopes mentioned above. On the basis of one of these parameters approximate scales of radiosensitivity were