OBJECTIVE: Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide are incretin hormones, secreted in response to meal ingestion. The incretin hormones stimulate insulin secretion and are essential for the maintenance of normal plasma glucose concentrations. Both incretin hormones are metabolized quickly by the enzyme dipeptidyl peptidase-IV (DPP-IV). It is well known that type-2 diabetic patients have an impaired incretin effect. Therefore, the aim of the present study was to investigate plasma DPP-IV activity in the fasting and the postprandial state in type-2 diabetic patients and control subjects. DESIGN: The study included two protocols. Protocol one involved 40 fasting type-2 diabetic patients (28 men); age 61 +/- 1.4 (mean +/- s.e.m.) years; body mass index (BMI) 31 +/- 0.6 kg/m(2); HbAlc 7.2 +/- 0.2%; and 20 matched control subjects (14 men) were studied. Protocol two involved eight type-2 diabetic patients (six men); age 63 +/- 1.2 years; BMI 33 +/- 0.5 kg/m(2); HbAlc 7.5 +/- 0.4%; eight matched control subjects were included. METHODS: In protocol one, fasting values of DPP-IV activity were evaluated and in protocol two, postprandial DPP-IV activity during a standard meal test (566 kcal) was estimated. RESULTS: Mean fasting plasma DPP-IV activity (expressed as degradation of GLP-1) was significantly higher in this patient group compared with the control subjects (67.5 +/- 1.9 vs 56.8 +/- 2.2 fmol GLP-1/h (mean +/- s.e.m.); P=0.001). In the type-2 diabetic patients, DPP-IV activity was positively correlated to FPG and HbAlc and negatively to the duration of diabetes and age of the patients. No postprandial changes were seen in plasma DPP-IV activity in any of the groups. CONCLUSIONS: Plasma DPP-IVactivity increases in the fasting state and is positively correlated to FPG and HbAlc levels, but plasma DPP-IV activity is not altered following meal ingestion and acute changes in plasma glucose.