Adult, juvenile, and tadpole Fowler's toads were exposed to graded doses of whole-body gamma radiation. Radiation mortality, time of lethality, and changes in behavior and development were studied for 50 days or more. The estimated <tex-math>${\rm LD}_{50/30}$</tex-math> for adults was 2329 rad, for juveniles uncertain but near 1000 rad, and for tadpoles 1670 rad. The estimated <tex-math>${\rm LD}_{50/50}$</tex-math> for adults was 1780 rad and for juveniles and tadpoles approximately 100 rad. At most lethal radiation doses, adults survived much longer than tadpoles, and tadpoles longer than juveniles. Irradiated tadpoles failed to metamorphose. Behavioral changes were noted in irradiated toads of all stages. Damage to hemopoietic tissues was the prominent pathologic feature in all three life stages. Skin necroses, hemorrhage, bacterial invasion of tissues, and atrophied ovaries were observed in irradiated adults. Irradiation effects in other amphibians are reviewed.

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