We studied the effect of partial ileal bypass in the rabbit on the in vivo catabolism of human 125I-labelled low density lipoproteins and on the in vitro binding of human low density lipoproteins and rabbit very low density lipoproteins to hepatic membrane preparations. The in vivo data indicate that partial ileal bypass increases the fractional clearance rate (pools/h) of low density lipoproteins from 0.031 to 0.049 as well as the absolute catabolic rate from 0.495 to 0.605 mg/h. Concomitantly the in vitro binding of both low and very low density lipoproteins to hepatic membranes was increased in membrane preparation from livers of bypassed animals, thus suggesting an increased receptor-mediated uptake of lipoproteins by the liver. This effect may partly explain the hypocholesterolemic activity of partial ileal by-pass.