The differences of serum lipid and lipoprotein (LP, profiles of animals susceptible (rabbit) and nonsusceptible (Beijing duck) to atherosclerosis as well as the distribution of apolipoprotein (apo) A-I and its catabolism in vivo were studied. Eight items, i.e. total cholesterol (TC), high density lipoprotein (HDL) cholesterol, triglyceride, percentage of alpha-LP and beta-LP, beta-LP concentration, lecithin cholesterol acyl transferase (LCAT), and agarose and polyacrylamide gel electrophoresis of both species were assayed before and after feeding a high fat, high cholesterol diet. Results indicate that the exogenous cholesterol consumed by Beijing ducks was carried and transported by HDL, while that in rabbits was transported by low density lipoprotein (LDL). The biological half lives of apo A-I in serum and in HDL were 41.1 +/- 2.4 and 42.8 +/- 1.7 h respectively, and its distribution in different organs was in the order of liver greater than kidney greater than spleen greater than lung greater than heart greater than intestine greater than muscles greater than aorta. These results show that the liver is the major organ for metabolizing HDL apo A-I, and the kidney is also important. The results also imply that in Beijing ducks the cholesterol carried by HDL may be catabolized through apo A-I receptors in the liver and kidney. The differences in cholesterol binding and in lipoprotein and apolipoprotein metabolism of the two species provide important clues for the elucidation of the pathogenesis of atherosclerosis.