We studied the high-density lipoprotein (HDL) metabolism of five trained men who ran 16 km daily and five inactive men. Runners were leaner and their aerobic exercise capacity was much greater. The mean HDL cholesterol level was 65 mg mg/dL in the runners and 41 mg /dL in the controls. The lipid-rich HDL2 species accounted for a much higher proportion of the HDL in runners (49% v 29%). Tracer studies of radioiodinated autologous HDL demonstrated that runners did not produce more HDL protein but rather catabolized less. The mean biologic half-life of HDL proteins was 6.2 days in the runners compared with 3.8 days in the sedentary men. The activity of lipoprotein lipase was 80% higher in the postheparin plasma of the runners, whereas the activity of hepatic triglyceride hydrolase was 38% lower. Thus, the prolonged survival of plasma HDL proteins in runners may result from augmented lipid transfer to HDL by lipoprotein lipase or diminished HDL clearance by hepatic lipase.