The authors' interest has been concerned with the effectiveness of physical training in the prophylaxis of arteriosclerosis. As an approach to the clarification of the mechanism underlying the preventive effect, investigations have been primarily conducted on the alteration in cholesterol metabolism caused by a long-term physical exercise. Male rats, 5-6 weeks of age and weighing approximately 100g, were trained for 5-6 weeks according to a treadmill running program. They were running at 12m/min for 60min/day, 6 days/week. At the end of the training program, the level of cholesterol in serum and liver of the trained and untrained (control) rats was determined. Simultaneously, the incorporation of ^<14>C-acetate into liver cholesterol was studied with both groups of animals. Ten μCi of sodium acetate-1-^<14>C were given intraperitoneally per 100g of body weight of rats. Animals were killed 90 minutes after the injection. Liver cholesterol was isolated and assayed for the radioactivity using a scintillation spectrometer. The average ± standard deviation of the total cholesterol in serum were 63.59±8.16 and 68.61±8.26mg/100ml in the trained and untrained rats, respectively. Those of the free cholesterol were 12.54±1.51 and 15.34±3.55mg/100ml, respectively. The trained rats showed a decrease in both total and free cholesterol of liver, compared with the untrained group. The amounts of total cholesterol were 4.62±0.30 and 4.73±0.43mg/g liver in the trained and untrained rats respectively, and those of free cholesterol were 1.03±0.11 and 1.16±0.17mg/g liver, respectively. In the trained rats the synthesis of cholesterol in liver was significantly stimulated compared with that in the control rats. The incorporated radioactivities were 1,254±190cpm/mg cholesterol in the trained rats, whereas 811±167cpm/mg cholesterol in the untrained rats. The results of this experiment demonstrates that the physical training produces an enhanced turnover of cholesterol, which would be favorable for protecting the accumulation of cholesterol esters in the wall of arteries.