The action of sucrose polyesters (SPE) as undigestible fat-like compounds on lipid metabolism of growing rats was studied. Over a period of 4 weeks the animals were fed a semi-synthetic diet containing 6 % of SPE with or without reduction of dietary fat. The substitution of dietary fats by SPE or the additional supply of SPE led to an increase of fecal lipid and cholesterol excretion, which was accompanied by a significant stimulation of in vitro cholesterol synthesis of the liver and by a significant reduction of liver cholesterol. The serum cholesterol level, however, was not affected, whereas the level of triglycerides both in serum and liver was significantly reduced to about 40% of that found in control animals.