The plasma concentrations of gastric inhibitory polypeptide (IR-GIP), insulin, and glucose were measured in 5 dogs after administration of various constituents of food. IR-GIP responses produced by galactose and fructose were minimal compared to those of glucose. Corn starch caused a significant early increase in insulin and glucose concentrations, although the rise in IR-GIP was delayed. The responses after starch were delayed, possibly due to the necessity for hydrolytic cleavage. Neither individual fatty acids (palmitic, oleic, and linoleic), monoolein, nor glycerol produced increases in IR-GIP comparable to those observed with lesser amounts (on a molar basis) of corn oil alone. None of four amino acids tested orally (arginine, leucine, alanine, and phenylalanine) caused increases in IR-GIP concentrations, but they increased insulin concentrations. Although glucose-induced IR-GIP responses were not inhibited by the amino acids (1 g/kg) tested, increasing concentrations (up to 1 g/kg) of either alanine or arginine caused progressive inhibition of corn-oil-stimulated IR-GIP responses, in spite of the expected amino-acid-induced rises in insulin concentrations. These results confirm the specificity of the IR-GIP secretory response for glucose and triglyceride, and suggest that certain food constituents (such as amino acids) may modify the GIP secretory response to a meal.