An anorexigenic substance (FS-T), found in feces, isolated and injected intraperitoneally, induced significant feeding suppression in Wistar rats and in genetically obese Zucker rats (fa/fa) and their lean littermates. The concentration of total plasma amino acids 2 h after FS-T injection (the time of maximum feeding suppression) was 71.0, 68.6 and 60.2% of that of controls for Wistar and Zucker obese and lean rats, respectively. By 48 h after injection of FS-T, food intake and the concentration of total plasma amino acids had returned to normal. Plasma tryptophan levels and the ratio of tryptophan to neutral amino acids were also monitored to elucidate the relation between FS-T and appetite. Two h after injection of FS-T, the ratio of tryptophan to neutral amino acids had increased in Wistar rats, while no change was detected in either obese or lean Zucker rats. However, no change was observed in plasma glucagon levels in Wistar rats, but a significant increase was found in both obese and lean Zucker rats at 2 h after FS-T injection.