The applicant is interested in the understanding of the morphology and physiology of the adipose organ at various stages of its development, in different species. It is now well established that the adipose organ has the unique capacity to continue to expand beyond the size reached at maturity of the animal and that during this expansion the ratio of lipid to protein in the tissue increases. In young growing rats and mice, the principal metabolic pathways of the fat cells, and their regulation by hormones of the pancreas, pituitary and adrenergic nervous system have been thoroughly elucidated. In contrast, only limited information is available on the metabolic organization of adipose tissue when this tissue expands. New techniques to characterize adipose tissue in terms of fat cell size and cell number have been developed. With these techniques, a meaningful comparison of the metabolic capacities of adipose tissue cells from animals of different body weight is now possible. The objectives of this research project are to understand in the rat, the human and other selected species: the contribution of fat cell hypertrophy and cell hyperplasia to the expansion of the adipose organ; the relationship between fat cell size, metabolic activities of the fat cell, and the hormonal responsiveness of the cell; and the consequences of the altered metabolism of the expanded adipose tissue on the extra-adipose regulation of carbohydrate and lipid metabolism.