Some amino acids are important regulators of key metabolic pathways and necessary for several physiological functions. However, little is know about thermoregulatory functions of amino acids. In this study, therefore chicks were either centrally or orally administered with L-citrulline (L-Cit), L-arginine (L-Arg) or L-ornithine (L-Orn) to monitor changes in rectal temperature. In Experiment 1, the amino acids (L-Cit, L-Arg and L-Orn) were administered into the left ventricle of the chicks by intracerebroventricular (i.c.v.) injection at a dose of 1 μmol/10 μl to monitor the effects of these amino acids on rectal temperature during 120 min of the experimental period. In Experiment 2, chicks received the same amino acids by oral administration at a dose of 15 mmol/10 ml/kg body weight. In Experiment 3, chicks received three doses of L-Cit (3.75, 7.5 or 15 mmol/10 ml/kg body weight) by oral administration. I.c.v. injection with any of the amino acids studied did not alter body temperature, but oral administration of L-Cit significantly reduced body temperature. Importantly, the highest does effectively reduced body temperature. These results suggest that peripheral L-Cit has a hypothermic function in chicks, which may be a new candidate to minimize high body temperature in poultry during summer heat stress.