The respiratory control and P/O ratios of isolated rat liver mitochondria held aerobically in buffered 0.3 M mannitol-0.2 mM EDTA in an ice bath decreased exponentially immediately after X-irradiation at 20,000 R, in close correlation with the decrease of the preirradiation respiratory control. Of the two indicators of mitochondrial intactness, the respiratory control was the more sensitive to irradiation. This radiation effect appeared to be similar to uncoupling. The losses of respiratory control and P/O ratio in the irradiated mitochondria were amplified by transfer to ice-cold buffered 0.15 M mannitol without EDTA; the degree of this amplification reached a maximum for values of the preirradiation respiratory control ratio in the neighborhood of 4.0. The nature and direct cause of the radiation damage in mitochondria are discussed.

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