An anorexigenic material isolated from rat urine (FMS IA) was investigated. Ultrafiltration and sieving through polyacrylamide gel indicated a molecular weight in excess of 50 000. Anorexigenic activity was not destroyed by heating to 37 °C for 6 h or to 100 °C for 10 min, or by digestion with pepsin for 2 h. Digestion with trypsin yielded an active fraction with molecular weight < 10 000. Extraction of this digest with ether did not eliminate the activity nor did the ether extract possess anorexigenic activity per se. Qualitative tests of the ether extract were negative for steroid-like material. These and prior results suggest the anorexigenic factor may be a polypeptide bound to a larger protein. In normal and hypothalamicobese rats, the amount excreted in urine was greater under fed than under fasting conditions. The obese rats were more responsive to the anorexigenic material than were normal rats. It is suggested that the anorexigenic factor may play a role in the regulation of food intake.