drawn from the vast majority of the data that, within strain, sex has a constant effect in the sense that female animals are more sensitive to radiation-—induced life-shortening, which effect is mostly manifested by an increased incidence of tumours of the female genital tract. 285. The data on the rat show no obvious difference of the sex, while the guinea-pig, in analogy with most data in the mouse, shows an increased sensitivity of the females attributable to tumours of the genital tract. Body weight might also be a biological variable to interest in the final expression of life~ shortening, but it appears to be of rather minor importance. 286. As to the effect of age, after allowing for other factors influencing the reduction in longevity (genetic background and sex) most data agree in showing that irradiation in utero of the mouse produces less marked life-shortening than irradiation during post-gestational ages. There may even be no long-term effect at all on the irradiated animals, particularly those surviving irradia- tion at the early gestational ages. The experience in the rat shows some re- duction of the life-span for irradiation of the foetal animals, but the effects observed are of doubtful significance and in any case not substantially different from the effects of the same doses given soon after birth. 287. Regarding the effect of extra-uterine age, the data are rather numerous but only limited to the mouse and the rat. In both these species invariably irradiation late in life produces - all other factors being equal - less lifeshortening than treatment at younger ages. In the one case where no effect of age was found [U5] the range of useful ages examined was too short for any effect to be seen. In some instances the reduction of life-shortening with age is preceded by a phase of increased susceptibility of the animals up to the time of sexual maturity {L12]. In other cases irradiation in old ages may even pro- duce (for moderately high doses) an increase, rather than a decrease, of the duration of life [Y3] or no change with respect to control [J3]. The change in sensitivity between young and old animals may be up to a factor of three when the life-shortening per unit dose is considered; if the effect is evaluated in terms of the percentage loss of the remaining life-span, this amounts to a few per cent. In some experiments a correlation may be established between the degree of life-shortening and the induction of tumours or of nephrosclerosis, but in other cases no such correlation may be found.