The purpose of this study was three-fold: 1) to define the effects of obesity and Type 2 diabetes on plasma lactate concentrations; 2) to relate changes in plasma lactate concentration to plasma glucose and insulin concentrations; and 3) to evaluate the effect of differences in blood sample processing on plasma lactate determination in a disparate population group. To accomplish this, fasting plasma lactate concentrations were determined in 30 volunteers (10 non-obese individuals with normal glucose tolerance, 10 obese individuals with Type 2 diabetes) on blood drawn, processed, and maintained in a variety of ways. Results demonstrated that fasting plasma lactate measurements were least confounded when blood was drawn without the subject "hand pumping" following venous occlusion, the samples maintained on ice at 4 degrees C until precipitated with perchloric acid, and kept as this temperature until lactate concentration was determined. Under these conditions, plasma lactate concentration was lowest in the non-obese group with normal glucose tolerance (0.81 +/- 0.07 mmol/L), highest in the obese subjects with Type 2 diabetes (1.46 +/- 0.14 mmol/L), and intermediate in obese individuals with normal glucose tolerance (1.17 +/- 0.13 mmol/L). All three groups were significantly different from each other. In addition plasma lactate concentrations were associated with both fasting plasma glucose and glycated haemoglobin concentrations.