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A survey of the metabolism of 95Zr and 95Nb by the rat has been made, and the results obtained have been used to reassess the radiological hazards of these isotopes. The main conclusions are: (1) The gastrointestinal absorption of both elements may be a factor of twenty times greater than the figures of 10-4 used by the International Commission on Radiological Protection (1). (2) Maximum permissible levels for both isotopes in insoluble forms, or dissolved in water, remain unaltered, being set by irradiation of the gastrointestinal tract by its contents. (3) Maximum permissible levels for soluble forms of both isotopes in air are not significantly changed, providing no account is taken of non-uniform distributions within organs, or of hazards to a foetus. (4) The unsuspected foetus could easily receive a dose greater than 1 rem in the first 2 months of pregnancy, and it is calculated that the maximum permissible levels for soluble isotopes in air need to be 3.3 × 10-8 μCi of 95Zr/cc of air, and 8.5 × 10-8 μCi of 95Nb/cc of air, to prevent this. (5) During the last 7 months of pregnancy the maximum permissible levels for soluble isotopes in air need to be 4.9 × 10-9 μCi of 95Zr/cc of air, and 10-8 μCi of 95Nb/cc of air, in order that the integrated dose to the foetus in this period shall not exceed 1 rem.