The effects of insulin injections (0.1, 1, 10 and 40 mU) into the paraventricular hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. The 0.1 and 1 mU doses had no effects on either respiratory quotient or energy expenditure. The 10 mU dose increased respiratory quotient which indicates increased dependency on carbohydrates as an energy substrate. The same dose had no effects on thermogenesis. In contrast, the 40 mU dose decreased respiratoy quotient which indicates increased dependency on fats as an energy substrate. The higher dose also increased thermogenesis. Since neither dose significantly affected locomotor activity, the metabolic data are not confounded with activity effects. These data indicate that insulin in the PVN produces a primary modulation of the metabolic parameters central to maintaining energy balance. In separate experiments, the 4 doses of insulin reduced food intake and body weight over a 24 h period. They also produced a dose-related increase in blood glucose concentration over a one hour period. Taken together, these findings are interpreted in a model in which insulin in the PVN acts as signal indicating increased body fat. This increases regulatory reduction in body fat.