Eleven healthy men and women (63 ±1 years) participated in a 12-month endurance-training program to determine the effects of low-intensity and high-intensity training on glucose tolerance and plasma lipids in older persons. Plasma glucose, insulin, and C-peptide concentrations were measured for three hours after ingestion of 100 g of glucose and the total areas under the respective curves were calculated. Total plasma lipids and lipoprotein concentrations were determined during fasting. Maximal oxygen uptake increased 12% during six months of low-intensity training; a further 18% increase occurred during an additional six months of high-intensity training. Glucose tolerance, which was normal initially, was not significantly changed after training. However, the total area for insulin was 8% lower after low-intensity training, and 23% lower after high-intensity training, compared with before training. C-peptide concentrations were similarly reduced. Plasma lipid and lipoprotein concentrations were unchanged after low-intensity training, but high-intensity training resulted in an increase in high-density lipoprotein cholesterol and a reduction in triglycerides. These results demonstrate that older persons respond to prolonged, high-intensity endurance training with an increase in sensitivity to insulin and a favorable alteration in their plasma lipoprotein-lipid profile.