The Prague Hereditary Hypercholesterolaemic (PHHC) rat is a strain of the Wistar rat very sensitive to dietary cholesterol.The dynamics of changes in serum and liver lipids and lecithin : cholesterol acyltransferase (LCAT) were studied immediatelly after the switch to a high cholesterol diet. Immediate cumulation of free and esterified cholesterol in the liver after the increase in alimentary cholesterol intake is supposed to be the regulating step leading to a subsequent increase in serum cholesterol concentration. Activity of LCAT was negatively correlated to the concentration of free cholesterol in the liver, very early after the cholesterol diet was introduced, a possibility of a down regulation of enzyme synthesis similarly to the regulation of synthesis of cholesterol in hepatocytes was observed.