Fat transplants increase body fat mass without changing the energy status of an animal and provide a tool for investigating control of total body fat. Early transplant studies found that small pieces of transplanted fat took on the morphology of the transplant recipient. Experiments described here tested whether this response was dependent upon expression of leptin receptors in either transplanted fat or the recipient mouse. Fat from leptin receptor deficient db/db mice or wild type mice was placed subcutaneously in db/db mice. After 12 weeks cell size distribution in the transplant was the same as in endogenous fat of the recipient. Thus wild type fat cells, which express leptin receptors, were enlarged in a hyperleptinemic environment indicating that leptin does not directly control adipocyte size. By contrast, db/db or wild type fat transplanted into wild type mice decreased in size, suggesting that a functional leptin system in the recipient is required for body fat mass to be controlled. In the final experiment wild type fat was transplanted into a db/db mouse parabiosed to either another db/db mouse to an ob/ob mouse or in control pairs in which both parabionts were ob/ob mice. Transplants increased in size in db/db-db/db pairs, decreased in db/db-ob/ob pairs and did not change in ob/ob-ob/ob pairs. We propose that leptin from db/db parabionts activated leptin receptors in their ob/ob partners. This in turn stimulated release of unidentified circulating factors which travelled back to the db/db partner and acted on the transplant to reduce fat cell size.