Plant sterols and their saturated derivatives, known as stanols, reduce serum cholesterol when consumed in amounts of approximately 2 g per day. Stanol fatty acid esters have been developed as a highly fat-soluble form that may lower cholesterol more effectively than stanols. Stanol esters occur naturally in human diets, but at levels far below those known to lower cholesterol. The present study was conducted to assess the safety of stanol esters upon subchronic ingestion at levels comparable to or exceeding those recommended for lowering cholesterol. Two stanol fatty acid ester preparations, wood-derived stanol esters and vegetable oil-derived stanol esters, were fed to groups of 20 male and 20 female Wistar rats for 13 weeks, at dietary concentrations of 0, 0.2, 1, and 5% total stanols (equivalent to 0, 0.34, 1.68, and 8.39% wood-derived stanol esters and 0, 0.36, 1.78, and 8.91% vegetable oil-derived stanol esters). Both preparations were well tolerated as evidenced by the absence of clinical changes or major abnormalities in growth, food and water consumption, ophthalmoscopic findings, routine hematological and clinical chemistry values, renal concentrating ability, composition of the urine, appearance of the feces, estrus cycle length, organ weights, gross necropsy findings, and histopathological findings. Plasma cholesterol and phospholipids were slightly decreased in males fed the stanol esters. In both sexes, plasma levels of plant sterols were decreased whereas those of stanols tended to increase. Fecal excretion of sterols, including cholesterol, and stanols was markedly increased in the stanol ester groups. Compared to controls, male rats fed stanol esters showed somewhat lower liver weights and more pronounced glycogen depletion. These hepatic changes were considered to reflect an altered nutritional condition and not a pathological condition. Plasma levels of vitamin E, vitamin K1, and, to a lesser extent, vitamin D were decreased in males and females fed the high-dose diets. Hepatic levels of vitamins E and D showed similar changes (vitamin K1 in the liver was not determined). For both preparations, the mid-dose level (1% total stanols in the diet) was a no-observed-adverse-effect level. This dietary level provided approximately 0.5 g total stanols/kg body wt/day.