The evidence that exercise reduces resistant to the action of insulin has generated interest in the possibility that endurance exercise training may be a worthwhile primary therapeutic intervention in the treatment of mild non-insulin-dependent diabetes mellitus (NIDDM). Unfortunately, the results of several studies of exercise training in NIDDM have been discouraging since improvements in oral glucose tolerance (OGT) did not occur or were quite modest even though insulin resistance was reduced. Recently it was determined that 12 months of endurance exercise training at 75-90% of VO2max could normalize OGT in men with mild NIDDM when the post-training OGTT was performed within 18 h of the last bout of exercise. This, coupled with the fact that the enhanced action of insulin is lost in trained persons within a few days of cessation of training, suggests that improved glucose tolerance in NIDDM patients may be partly due to the persistent effects of the last bouts of exercise. Acute exercise in the form of 7 d of intense walking/cycling has been shown to improve glucose tolerance despite a significantly smaller increase in plasma insulin levels during the OGTT in NIDDM. Apparently, the improvement in OGT was due to a decrease in resistance to insulin over the short term since no changes in body weight, body fat, or VO2max took place. Thus, acute exercise of sufficient intensity and duration can increase peripheral insulin action and may contribute to the effects of long-term exercise training on improvement in OGT and the amelioration of insulin resistance in patients with NIDDM.