The substitution of sucrose for starch in the diet of the rat promotes a rise in the plasma cholesterol concentration which is largely confined to the esterified fraction. There is a concomitant rise in the activity of the enzyme lecithin:cholesterol acyltransferase which catalyses the esterification of free cholesterol in the plasma. This enzyme displays a marked specificity towards the cholesterol carried in the high-density lipoproteins. It is concluded that the increase in the capacity of the plasma to esterify cholesterol occurs in response to the increased throughput of cholesterol in the plasma which results from feeding a high-sucrose diet.