Special rat strains are maintained and made available for study here and elsewhere, notably the hereditarily obese rat "fatty", the Zucker rat for the study of problems of obesity, and our large and small strains obtained by two-way selection for body size for problems of size and growth. The hereditarily obese rat constitutes an excellent model for obesity studies. It is of convenient size for experimental work. It is non-diabetic. The obesity does not depend on any particular type of diet for its production, and resembles common forms of human obesity in its patterns of fat mobilization and utilization, plasma insulin, and response to reducing diets of varying compositions. Large differences in growth probably interact with endocrine constitution, with necessary consequences of differences in patterns of metabolic activity and regulation. This proposition can be explored by means of two rat strains of very large and very small size which should be of closely similar genetic constitution except for genes interacting with size. The 3 strains constitute experimental material suitable for the study of differences in the main metabolic flowsheet as between growth. energy utilization and storage, and particularly for the elucidation of growth hormone action.