Parabiosis is the surgical union of two animals to produce a chronic blood exchange. This model has previously been used to demonstrate the involvement of a blood-borne factor in the feedback control of food intake and regulation of energy balance. It has been hypothesized that obese rats produce a humoral agent that acts centrally to inhibit food intake and accumulation of fat. In this study 50-day-old male or female Zucker rats were joined in either lean-lean pairs or lean-obese pairs. They ate ad libitum until 152 days of age when body composition was determined. Parabiosis inhibited growth in all rats compared with single controls. Lean partners of obese rats had reduced carcass weights, the same percent body protein but less fat than members of lean-lean pairs. Female rats showed larger changes in body composition than did males. These results suggest that obese Zucker rats produce the hypothesized regulatory signal but do not respond to it.