Summary: The sizes of cholesterol‐containing lipid particles were, on the whole, larger in thoracic duct lymph of rabbits and rats fed a cholesterol‐high fat diet than when fed a cholesterol‐low fat diet. The mean diameter of chylomicrons of Sf > 400 from rabbits fed a diet containing 0.8% cholesterol‐30% corn oil was 1476 Å, while that from rabbits fed 0.8% cholesterol in plain food was 1065 Å. The mean diameters of the lipoproteins of Sf12–400 were 493 and 499 Å respectively. Not only were chylomicrons larger in the cholesterol‐high fat diet group of rabbits, but cholesterol was distributed to a greater extent in this fraction. In lymph of the cholesterol‐low fat diet group, on the other hand, relatively more cholesterol was present in the smaller lipoproteins of Sf12–400.A comparison of the distribution of cholesterol estimated chemically with that of radioactivity following the ingestion of cholesterol‐4‐14C suggested that the small lipoproteins of D > 1.019 were to a large extent derived by filtration from the serum and that some of the cholesterol of Sf12–400 was derived from sources other than recently ingested cholesterol, while a high proportion of cholesterol of chylomicrons was derived from food.