Ten normal subjects were given 50 g starch, or 50 g starch + 50 g fat as a breakfast meal. The starch was given in the form of potato; the fat was given in the form of butter. The meals were ingested at 8 a.m. Plasma glucose, insulin, C-peptide, and triglyceride concentrations were measured at various time points for 4 hours after each meal. The net 4-hour postprandial area responses to the ingested meals were determined using the trapezoid rule, with the fasting glucose concentration, measured at the same time points for 4 hours as a baseline. The glucose area response was 2.2 mmol hour/l following the potato meal. This was significantly reduced following ingestion of the meal containing fat (1.3 mmol hour/l) (p < 0.01). The insulin area response was slightly greater following the meal containing fat (459 pmol hour/l) compared to potato alone (423 pmol hour/l) (p < 0.01). The C-peptide area response following the meal containing fat was 0.80 pmol hour/ml, clearly greater than following potato alone (0.58 pmol hour/ml) (p < 0.01). The triglyceride area response also was much greater following the meal containing fat compared to potato alone (0.74 and 0.08 mmol hour/l, respectively). The mechanism of the attenuated glucose response to carbohydrate ingestion with a fat-containing meal is unknown. It may be due to the release of an enteric hormone that increases glucose disposal, either directly or indirectly, through insulin.