The purpose of this study was to determine the relationship of mitochondrial proliferation and ATP production to milk production in two lines of mice that were genetically divergent for milk production. Milk production differed between high production and low production lines by .8 phenotypic standard deviations as determined by cross-fostered litter weight gain from 1 to 12 d postpartum. Mammary weight, mammary total DNA, and RNA:DNA ratio were greater in glands of high line mice. Mammary DNA and protein, expressed per gram mammary tissue, were similar between lines. Mammary mitochondrial mass per gland differed after six generations of divergent selection. Rates of succinate-supported ATP production and ADP:O of isolated mitochondria differed, but the rate of pyruvate-supported ATP production did not differ between lines. Differences between selection lines in mitochondrial mass and in the efficiency of succinate use for support of ATP production were probable consequences of selection for divergent milk production.