The purpose of the present study was to determine the effects of exercise training on the chemical composition of plasma low-density lipoprotein (LDL). Thirteen men (mean age +/- SE, 47.2 +/- 1.5 years) were examined before and after 14 weeks of endurance-oriented exercise training (3 to 4 d/wk, 30 to 45 min/d). Although calculated plasma LDL concentrations remained unaltered (3.49 +/- 0.24 versus 3.65 +/- 0.23 mmol/L), changes in the chemical composition of LDL (increased LDL free cholesterol, cholesterol ester, and phospholipid content) were associated with a reduction in adiposity, umbilical girth, and basal plasma insulin and glucose concentration with training intervention. Increases in LDL molecular weight and particle diameter were associated with a reduction in fat mass, plasma triglyceride concentration, and basal plasma glucose concentration with physical activity. The LDL lipid-to-protein ratio also increased (P < .01) with training by 7%, primarily due to an increase in LDL free cholesterol content (P < .01). These findings indicate the formation of LDL particles that are more cholesterol enriched and protein poor with exercise training, which provides additional evidence for the cardioprotective effect of long-term physical activity.