The role of physical activity in the prevention of non-insulin-dependent diabetes mellitus (NIDDM) is of utmost importance. The aim of the present study was to evaluate the metabolic effects of aerobic endurance exercise and circuit-type resistance training in subjects with impaired glucose tolerance (IGT). Twenty-two individuals participated in the study. Fourteen subjects were enrolled in the aerobic endurance exercise part of the study; seven exercised regularly for six months, while seven served as controls. Maximal aerobic capacity (VO2max) was measured and insulin sensitivity and insulin secretion were assessed by a frequently sampled intravenous glucose tolerance test (FSIVGTT). Eight subjects participated in a circuit-type resistance training program for three months. Insulin sensitivity and substrate oxidation were then assessed using the euglycemic insulin clamp technique combined with indirect calorimetry. The aerobic endurance exercise program caused in increase in VO2max (21.6 +/- 1.9 to 25.4 +/- 2.4 ml/kg.min; p < 0.05) and HDL-cholesterol (1.14 +/- 0.06 to 1.23 +/- 0.08 mmol/l; p < 0.05), but no change in insulin sensitivity nor insulin secretion occurred. However, comparing the changes between the intervention and control group, the differences disappeared. Circuit-type resistance training increased insulin sensitivity (glucose disposal) by 23% (p < 0.05), primarily due to a 27% increase in non-oxidative glucose metabolism. Both circuit-type resistance training and aerobic endurance exercise seem to have beneficial effects in subjects with impaired glucose tolerance. However, by improving insulin sensitivity, circuit-type resistance training may postpone the manifestations of NIDDM in these high-risk individuals and should therefore be included in an exercise program for IGT subjects.