Because of the body's resistance to permanent weight change, obesity remains a major health problem in modern society. It is hypothesized that the regulatory system defending body weight utilizes pancreatic insulin as an indicator of adiposity to the brain. To take advantage of this negative feedback system, we transplanted neonatal (experiment 1) or adult (experiment 2) pancreatic islets containing insulin-secreting cells into the 3rd ventricle of syngeneic Lewis rats. This resulted in an elevation of the insulin signal within the brain and a significant long-term reduction of body weight. Changes in food intake were consistent with the changes of body weight. The implantation of more islets resulted in a greater reduction of body weight, and changes in weight were inversely correlated with the level of insulin achieved in the cerebrospinal fluid. After the two studies were completed, histological examination revealed the presence of insulin-containing cells within the 3rd ventricle and adjacent hypothalamus. These studies suggest that transplanted insulin-secreting cells may provide a potential therapeutic strategy for maintenance of weight loss.