The mechanism responsible for weight stability in adult hamsters was investigated by (a) transecting the dorsoventrally oriented nerve pathways between the septal area and hypothalamus (SH cuts) and (b) partitioning the observed increases in the rate of weight gain into three contributory components: changes in somatic growth, in body fatness, and in energy expended as voluntary activity on horizontal disks. Between 60% and 70% of the weight increase after SH cuts was due to acquisition of lean body mass, and 30%-40% of weight increase consisted of excess body fat. After SH cuts, serum growth hormone and insulin concentrations were increased on Day 14, food intake was increased between Day 2 and Day 42, skeletal lengths were greater on Day 77, and voluntary activity levels were 84% lower on Days 10-45, relative to control hamsters. It is concluded that dorsoventrally oriented nerve pathways in the septal area are involved in the control of growth, maintenance of body fat reserves, and voluntary activity and that they contribute to the maintenance of stable body weight in adult hamsters.