Mitochondrial permeability was estimated enzymatically by the spectrophotometric estimation of the rate of formation of DPNH from exogenously added DPN, and manometrically with phenazine methosulphate, by intramitochondrial dehydrogenases. Permeability was unaffected by 9000 r in vitro or after 4 hours subsequent to whole-body X-irradiation with 1000 r. The osmotic and nonosmotlc swelling and the effect of phosphate, calcium and thyroxine were uninfluenced by in vitro irradiation. Mitochondria from liver, kidney, spleen and thymus of a variety of mammalian species were investigated.