Glucagon-like peptide-1 (GLP-1) is secreted by L cells in the small intestine in response to food ingestion. The influence of supplementation to diet with the amino acids, methionine (Met) and lysine (Lys), on the L cells in chicken small intestine was investigated using immunohistochemical and morphometrical techniques. Many endocrine cells showing immunoreactivity for GLP-1 antiserum were observed in the control, crude protein (CP) 0%, CP 0% + Met and CP 0% + Lys groups. The GLP-1-immunoreactive cells in all the groups were “open-typed” endocrine cells as viewed under light and electron microscopes. Differences in the shape and distributional pattern of GLP-1-immunoreactive cells were not observed between the control and experimental groups. Frequencies of the occurrence of GLP-1-immunoreactive cells in the CP 0% + Met and CP 0% + Lys groups were significantly lower than that of the CP 0% group, but significant differences were not recognized between the control group and the CP 0% + Met and CP 0% + Lys groups. Secretory granules in the control group were round to oval in shape. Elongated secretory granules were observed in the experimental groups, but not in the control group. Ratios of GLP-1-immunoreactive cells with elongated secretory granules in the CP 0% + Met and CP 0% + Lys groups were decreased compared with that in the CP 0% group. The size of round secretory granules in the control group was larger than that in all the groups. However, sizes of round secretory granules in the CP 0% + Met and CP 0% + Lys groups were larger than that in the CP 0% group. These morphological results indicate that amino acids may be a signal that influences on the secretion of GLP-1 in chicken small intestine.