Newborn (0-24 hr old), weanling (21 days old), and adult (4 mo old) rats were injected intravenously with doses of monomeric or polymeric239 Pu over the range of 6-90 μCi/kg. Some animals from each age-dose-form group were killed at intervals for determination of distribution and retention while the remaining rats were held for observation of acute mortality and several measures of delayed change. The most notable difference in partition involved a failure of the newborns to differentiate between the two physicochemical forms and an increased skeletal deposition in the weanlings exposed to monomer. As a result of these differences in deposition, as well as differences in retention and dilution, the cumulative radiation doses differed for the three ages. On an injected dose basis, monomeric239 Pu was more acutely toxic to newborn rats, <tex-math>${\rm LD}_{50/30}=63\ \mu {\rm Ci}/{\rm kg}$</tex-math>, than to adult or weanling rats, <latex>${\rm LD}_{50/30}=>90\ \mu {\rm Ci}/{\rm kg}$</latex>. Polymeric239 Pu was more acutely toxic to adult and weanling rats, <tex-math>${\rm LD}_{50/30}=47\ \mu {\rm Ci}/{\rm kg}$</tex-math>, than to newborn rats, <tex-math>${\rm LD}_{50/30}=77\ \mu {\rm Ci}/{\rm kg}$</tex-math>. Delayed deaths, between 90 and 270 days after injection, were more frequent among adults injected with monomeric239 Pu at levels of 6-90 μCi/kg (14/41, 90-day survivors) than among adults injected with polymeric239 Pu (6/25, 90-day survivors) or among either weanling or newborn rats given either form of239 Pu. Delayed effects such as mortality and mammary tumor incidence appeared to be influenced by the pattern of radiation doses associated with the physicochemical form administered and the age at exposure.

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