Mitochondrial and nuclear counts and homogenate and washed mitochondrial qO2 determinations were made on kidney cortex, liver and brain of albino rats. Homogenate qO2 was inversely related to mitochondrial count in the three tissues, but directly related to cellularity and to O2 uptake per mitochondrion. It is shown that the former is a spurious and the latter a real correlation. It is concluded that the characteristic differences in the qO2 of these three tissues are due largely to specific differences in the oxidative metabolism of their respective mitochondria and not to differences in the mitochondrial count per unit weight of tissue.