To assess the effect of exercise on the plasma concentration of cholesterol ester transfer protein (CETP) and its possible influence in mediating the exercise-associated redistribution of cholesterol among plasma lipoproteins, we measured plasma CETP in 57 healthy normolipidemic men and women before and after 9 to 12 months of exercise training. The training protocol resulted in significant changes in Vol., (mean±SD, +5-3±3.5 mL &#149; kg"1&#149; min"1), body weight (-2.5±3.5 kg), plasma triglycerides (-25.7±36J mg/dL), high-density lipoprotein cholesterol (HDL-C) (+2.6±62 mg/dL), and ratios of total cholesterol to HDL-C (-0.30±0.52) and low-density lipoprotein cholesterol (LDL-C) to HDL-C (-0.18±0.45) (all P$.05) but no change in lipoprotein(a). CETP concentration (in milligrams per liter) fell significantly in response to training in both men (n=28, 2.47±0.66 to 2.12±0.43; % A=14.2%;P<.005) and women (n=29, 2.72±1.01 to 2-J6±0.76; % A=13.2%;P<M7). The CETP change was observed both in subjects who lost weight (n=28, A mean weight=−5.0 kg; A CETP=−0.42±0.79; % A=15.4%;p<.009) and in those who were weight stable (n=29, A mean weight=−0.12 kg; A CETP =−0.29±0.78; % A=10.4%;p<.055). Pretraining plasma CETP concentration predicted training-associated changes in HDL-C (r= −.27,P<M) and ratio of LDL-C to HDL-C (r=+.40,p<.002). In a smaller study of 15 men, exercise training was associated with a decrease in levels of CETP, an increase in plasma postheparin lipoprotein lipase (LPL) activity, and a decrease in hepatic trigfyceride lipase (HTGL) activity. Overall, the data suggest that basal plasma CETP concentrations, in addition to LPL and HTGL activities, may contribute to determining the extent to which exercise redistributes cholesterol among plasma lipoproteins.