In summary, a number of different mechanisms likely exist to explain the modulation of LDL receptor activity in vivo by different dietary fatty acids. Our results provide direct evidence for one such pathway, i.e. unsaturated fatty acids increase cellular LDL receptor activity by an initial, direct stimulation of cellular ACAT activity. The depletion of free cholesterol from an, as yet, unidentified intracellular regulatory pool (to form cholesteryl ester) would then signal the cell for a need of increased exogenous cholesterol and a subsequent increase in LDL receptors. This may be one mechanism whereby certain unsaturated fatty acids increase LDL receptor activity in vivo, resulting in decreased plasma total and LDL cholesterol and reducing the risk of atherosclerosis.