Plasma lipids and chemical, electrophoretic and electron microscopic properties of VLDL, LDL and HDL are examined in rabbits fed a control diet (group I) or diets containing 1% cholesterol (group II), 1% cholesterol + 5% coconut oil (group III) or 1% cholesterol + 5% corn oil (group IV). The diets II, III and IV resulted in hypercholesterolemia, hypertriglyceridemia and hyperphospholipidemia. The lipid-protein composition of VLDL, LDL and HDL is changed by these diets. There is marked increase in the total cholesterol content of all lipoprotein fractions of the high fat dietary groups II, III and IV. The electrophoretic mobilities of the VLDL and LDL II and III are reduced while the respective mobilities in the corn oil group IV are nearly "normal". In contrast to the control LDL fraction I which is not precipitated by heparin, the LDL fractions of the dietary groups II, III and IV are readily precipitated. The apoprotein pattern of the lipoproteins in polyacrylamide gel differs distinctly between the dietary groups, most bands appearing in group IV. An abnormal stacking of lipoprotein particles in electron micrographs of VLDL, LDL and HDL of groups II and III can be observed. In contrast, these lipoprotein fractions of rabbits of the corn oil group IV have morphological properties that are similar to those of the lipoproteins of the control group. It is suggested that these findings are related to the marked reduction of atherosclerosis in rabbits fed a diet with polyunsaturated fat as compared with rabbits on cholesterol and cholesterol-coconut oil diets.