Large granule fractions, containing about 80% of the cytochrome oxidase of the tissue, were isolated from rat liver and used to prepare thin pellicles of packed particles which were submitted to quantitative electron microscopic examination. Various parameters describing the mitochondrial population were determined by measuring the size and number of mitochondrial profiles in sections, and the ratio of the inner to the outer membrane area. The mean particle radius and volume were found to be respectively 0.38 µ and 0.29 µ3; the average areas per mitochondrion were 2 and 5 µ2 for the outer and inner membranes respectively. On the basis of the cytochrome oxidase activity recovered in the particulate fractions, the results were extrapolated to the whole liver, and it was concluded that rat liver contains about 5.1011 mitochondria per gram; this corresponds to a volume of 0.14 ml/g and to an area of 2.5 and 1 m2/g for the inner and outer membranes respectively. The validity and the accuracy of these determinations is discussed and the results are compared to the information which has been obtained by independent methods or by other investigators.

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