Rabbits were fed either 10% coconut oil, 10% coconut oil and 1% beta-sitosterol, 10% coconut oil and 1% cholesterol, or 10% coconut oil and 1% beta-sitosterol plus 1% cholesterol for 4 weeks. Microsomal membranes from intestines of animals fed the 1% beta-sitosterol diet had 48% less cholesterol and were enriched twofold in beta-sitosterol compared to membranes from animals fed the coconut oil diet alone. Acylcoenzyme A:cholesterol acyltransferase (ACAT) activity in jejunum and ileum was decreased significantly in animals fed the plant sterol alone. In membranes from animals fed 1% beta-sitosterol and 1% cholesterol, beta-sitosterol content increased 50% whereas cholesterol was modestly decreased compared to their controls fed only cholesterol. Intestinal ACAT was unchanged in the animals fed both sterols when compared to their controls. beta-Sitosterol esterification was determined by incubating intestinal microsomal membranes with either [(14)C]beta-sitosterol-albumin emulsion or [(14)C]beta-sitosterol:dipalmitoyl phosphatidylcholine (DPPC) liposomes to radiolabel the endogenous sterol pool. Oleoyl-CoA was then added. The CoA-dependent esterification rate of beta-sitosterol was very slow compared to that of cholesterol using both techniques. An increased amount of endogenous microsomal beta-sitosterol, which occurs in animals fed 1% beta-sitosterol, did not interfere with the stimulation of ACAT activity secondary to cholesterol enrichment of the membranes. Enriching microsomal membranes three- to five-fold with beta-sitosterol did not affect ACAT activity. Freshly isolated intestinal cells were incubated for 1 hour with [(3)H]oleic acid and beta-sitosterol:DPPC or 25-hydroxycholesterol:DPPC. Incorporation of oleic acid into cholesteryl esters did not change in the presence of beta-sitosterol but increased fourfold after the addition of 25-hydroxycholesterol. We conclude that the CoA-dependent esterification rate of cholesterol is at least 60 times greater than that of beta-sitosterol. Membrane beta-sitosterol does not interfere with nor compete with cholesterol esterification. Inadequate esterification of this plant sterol may play a role in the poor absorption of beta-sitosterol by the gut.-Field, F. J., and S. N. Mathur. beta-Sitosterol: esterification by intestinal acylcoenzyme A:cholesterol acyltransferase (ACAT) and its effect on cholesterol esterification.