The transplantable Leydig cell tumor of Wistar rats, LTW(m), caused decreased food consumption and weight loss in the host within 2 weeks of implantation. The tumor was small, did not metastasize, and did not affect several parameters of biochemical function. When the tumors were removed, increases in food intake and body weight occurred within 72 hours and were sustained. Reimplantation of tumors caused anorexia to recur. Parabiotic pairs of rats with tumor in one partner also experienced weight loss. Those rats in parabiosis with tumor-bearing rats gained less weight than those in parabiosis with control rats. These observations suggest that the LTW(m) tumor causes anorexia and that this anorexia is mediated by a circulating substance.