1. Using quantitative electron microscopy, the volume fraction of mitochondria has been studied in the fibrillar and the perinuclear zones in leg muscle from young and old healthy men, physically trained and untrained, and from alcoholics with serious muscle weakness.

2. There is a correlation between the volume fractions of mitochondria in the fibrillar and perinuclear zones.

3. In young untrained men, the volume fraction of mitochondria in skeletal muscle is three-quarters of that in old untrained men in both the fibrillar and the perinuclear zone. In the young men, there are few but comparatively large mitochondria, while in old men the mitochondria are small but numerous.

4. Training causes a considerable increase in the volume fraction of mitochondria in young men (approx. 100%) but only a slight increase in old men (approx. 20%). The increase in the young group is caused by an increase in the number of mitochondria without any change of their volume, but in old men it is due to an increase in size.

5. Alcoholics with serious muscle weakness in their legs have a volume fraction of mitochondria which is below that of the healthy untrained controls. The decrease is of the same order as the increase induced by training in the old healthy men.

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