The effect of age, sex, and breeding on oxygen consumption and response to p-phenylenediamine of normal rat mammary tissue has been studied. Oxygen consumption values [Formula: see text] are significantly higher for breeder females than for virgin females or males. The endogenous oxygen consumption by breast tissue of virgin rats more closely resembles that of males than of breeder females. There is a significant lowering of [Formula: see text] with age in all the animals. Oxygen consumption in the presence of p-phenylenediamine [Formula: see text] is not significantly different for mammary tissue of males and breeders of the youngest age group (12–25 weeks) but it is higher for virgins of the same age. A highly significant fall in [Formula: see text] to approximately equal levels appears at a comparatively early age (30–50 weeks) in males and virgins, but this does not occur in the breeder female rats until the ability to reproduce has diminished or ceased (50 weeks and over). Reserves of the cytochrome system [Formula: see text] are lower in breeders than in males or virgins and are decreased by age in the latter two groups. These studies indicate the importance of considering age, sex, and breeding history of animals used for mammary tissue respiratory studies.
