Recently, we have reported that central administration of glucagon-like peptide-1 (GLP-1) strongly decreased food intake of chicks. The aim of the present study was to elucidate whether suppressed food intake by central injection of GLP-1 would be modified by an appetite stimulant such as fasting and neuropeptide Y (NPY). Birds (2 days old) were starved for 3 or 6 h and then GLP-1 (0.03 microg/10 microl) or saline was injected by the intracerebroventricular (i.c.v.) route. Birds starved for 6 h ate significantly more food than those starved for 3 h, while irrespective of the time for fasting GLP-1 strongly inhibited food intake as rapidly as 10 min after i.c.v injection. The suppressive effect on food intake continued until 4 h after injection. Central administration of NPY (2.5 microg/10 microl) greatly enhanced food intake, but co-injection of GLP-1 (0.01, 0.02 or 0.03 microg/10 microl) decreased food intake in a dose-dependent fashion. Under GLP-1 (0.03 microg/10 microl) treatment, whether NPY modifies food intake of chicks in a dose-dependent manner was investigated by co-injection of graded levels of NPY (0.4, 1.0 and 2.5 microg/10 microl). GLP-1 completely inhibited the effect of NPY on food intake without a dose response. These results suggest that central GLP-1 may interact with NPY and may be the most potent inhibitor of food intake in the chicken.