This project is designed to evaluate the developmental patterns of mammary glands of rats and mice fed diets containing different levels of saturated and unsaturated fatty acids. In addition, we wished to establish an atrophied mammary gland similar to that of immature animals but in an "adult" animal. The subsequent impact of manipulation of hormones and dietary lipid intake on mammary tumor incidence is also to be evaluated. Studies include: 1) Examination of the morphology of mammary glands of rats and mice at various ages after being fed EFA-deficient and control diets; 2) determination of the impact of periods of EFA-deficiency in rats on subsequent mammary tumor induction by NMU; 3) evaluation of the effects of reversal from EFA-deficient to control diet on formation of hyperplastic alveolar nodules (HAN) and ultimately on mammary tumor formation in mice; 4) determination of hormonal responsiveness of mammary glands of EFA-deficient mice in organ culture.