This research proposes to investigate reasons for persistent effects on growth and behavior caused by malnutrition of neonatal rats. Stunting will be induced by overcrowding (large litters), by artificial feeding, by periodic separation from a lactating female and by treatment with cortisone. Stunted animals will be compared to like-sex controls in several respects: 1) Spontaneous food intake (gm per 100 gm body weight) for any effect of conditions of nursing on appetite. 2) Behavioral competence, sensory-perceptual organization and cognitive functioning as measured by tests of cage orientation, visual discrimination, maze-learning and reasoning. 3) Metabolic effects (balances of calories, nitrogen, calcium, phosphate, magnesium, sodium and potassium) will be determined for absorption and utilization of nutrients from weaning throughout "rehabilitation". 4) Physical growth will be examined at several ages by measurement of cell size, cell number, length of individual bones and skeletal maturation (number of ossification centers or epiphyseal fusions found in cleared and stained skeletons). 5) Brain morphology will be examined. 6) The possibility that the effects can be prevented or repaired will be examined by hormonal treatment (growth hormone or androgens) at several ages. The appetite, behavior, metabolic balance, physical growth and skeletal maturation of animals given hormonal treatment will be compared to those of stunted litter-mate controls.