The survival of four species of amphibians (Desmognathus fuscus, Taricha granulosa, Amphiuma means, and Necturus maculosus) was followed after acute x irradiation. Survival parameters were determined from the literature for three additional species (Rana pipiens, Notophthalmus viridescens, and Ambystoma mexicanum). Nuclear volumes (NV) and interphase chromosome volumes (ICV) were determined from measurements of fixed and sectioned liver cell nuclei. An exposure of 1000 R was 100% lethal for Desmognathus, Taricha, Amphiuma, and Necturus, and 94% lethal for Rana. However, when the latent periods, preceding the time when mortalities increase sharply, were compared by species, it was found that they increased with increased nuclear and/or chromosome volumes. Median survival times after 1000 R also increased with increasing nuclear and/or chromosome volumes. The${\rm LD}_{50/30}$ values for Rana, Desmognathus, and Notophthalmus, plotted against ICV, suggest that species with larger chromosomes are progressively more resistant. However, when LD50 values were determined at longer times (until radiation-induced deaths ceased), LD50 decreased with increasing nuclear and chromosome volumes, as previously found for higher plant species. The LD50 for Rana was 724 R at Day 50, and for Desmognathus 530 R at Day 70. Necturus, with the largest nuclear and chromosome volumes of any amphibian known to us, is the most radiosensitive vertebrate thus far investigated, having an LD50 of 112 R at 170 days. However, extrapolation of existing data suggests that its LD50 at longer times may fall appreciably below this value, i.e., to 76 R at 200 days.

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