Body adiposity is normally maintained within rigid limits^1–3. Although it is not clear that this regulation fits a strict negative feedback pattern, animals do maintain a relatively constant body adiposity⁴. It has been postulated that this regulation is mediated by some signal which informs centres controlling food intake, probably located in the brain, as to the present state of adiposity^5,6. The identity of the signal is unknown, but the direct correlation between body adiposity and basal insulin levels in the plasma^7–9, suggests insulin as a possible candidate. This hormone is present in the cerebrospinal fluid (CSF) of many species^10–13, and is a slow integral over time of the level within the plasma¹⁴. Thus, the level of insulin in the CSF is relatively resistant to short-term plasma fluctuations of insulin. Obese humans have higher levels of CSF insulin than lean controls and the CSF insulin level of both obese and lean humans is reduced proportionately after a prolonged fast¹⁵. We have therefore postulated¹⁶ that the feedback system responding to body adiposity uses the concentration of insulin in the CSF as a major signal. Additional support for such a role is found in recent reports that insulin receptors are present in several regions of the brain and spinal cord^17–20. We now present additional evidence for our hypothesis by showing that in baboons the infusion of exogenous insulin into the CSF elicits a reliable and predictable decrease in food intake and body weight.