Apart from fasting blood glucose (FBG) and insulin (FBI), oral glucose tolerance test (OGTT) is also used in calculating insulin sensitivity. During OGTT, insulin secretion may not reflect normal physiological insulin secretion. Based on this idea, hepatic and whole body insulin sensitivity rates were tested during OGTT and mixed meal test (MMT) in obese subjects. Thirty-one women with Quantitative Insulin Sensitivity Check Index (QUICKI) values below 0.350 and body mass index (BMI) >or=30 were included into the study. OGTT with 75-g glucose and MMT 300 kcal were applied to all cases. Data obtained from OGTT and MMT were used in the assessment of insulin sensitivity with Hemostasis of Model Assessment-Insulin Resistance (HOMA-IR) and Matsuda's Composite Whole Body Insulin Sensitivity Index (Matsuda's ISI). Mean BMI, FBG, and FBI were 36.8 +/- 3.9 kg/m(2), 100.5 +/- 0.10 mg/dl, 16.2 +/- 5.3 microg/ml, respectively. QUICKI was 0.31 +/- 0.01 and HOMA-IR was 3.71 +/- 0.88. Matsuda's ISI derived from OGTT was 6.96 +/- 3.35 and from MMT was 11.32 +/- 6.61. In analysis, it was demonstrated that there was a correlation between HOMA-IR, QUICKI, and Matsuda's ISIs derived from OGTT and MMT. Comparing the time periods separately, it was detected that despite similar increment in insulin levels, glucose levels were higher in OGTT than MMT at 15 and 30 min. Consequently, Matsuda's ISI was demonstrated to be effectively used with the data of MMT, as used with OGTT. Moreover, MMT was shown to be in parallel to physiologic insulin secretion and reflect pancreatic functions better compared to OGTT.