The gut hormone glucagon-like peptide-1 (7-36) amide (GLP-1) is a potent insulin secretagogue. It has been proposed to be a novel treatment for non-insulin-dependent diabetes mellitus (NIDDM). Postprandial plasma GLP-1, insulin, and glucose responses were measured in six healthy volunteers in response to a solid test meal and a liquid meal of identical composition. Responses to three isocaloric soups of identical macronutrient and energy content containing differing degrees of fat saturation were also measured. The liquid form of the meal released significantly more GLP-1 than the solid form (measured by incremental area under the curve 0-180 min: 2.5 nmol.min-1.L-1 [median]; range 1.4-3.7 versus 1.4 nmol.min-1.L-1 [median]; range 0.6-1.8) (P < 0.05) and this occurred earlier (15 min versus 60 min). The incremental area under the curve for insulin was significantly greater following the liquid meal (incremental area under the curve 0-180 min: 18.5 nmol.min-1.L-1 [median]; range 15.9-35.8 versus 17.6 nmol.min-1.L-1 [median]; range 13.7-25.5) (P < 0.05). The glucose response to each meal was not different. The type of fat in the soups produced no significant difference in GLP-1, insulin, or glucose levels. Our findings suggest that the physical form of a meal significantly alters the GLP-1 response, whereas fatty acid saturation has little effect.