The effect of rapeseed oil with and without added squalene was studied on serum lipids and cholesterol metabolism. Dietary rapeseed oil reduced LDL cholesterol by 10%, increased cholesterol precursors and plant sterols, and decreased cholestanol in serum during a 6-wk baseline period from initial values. Addition of 1 g squalene in rapeseed oil for 9 wk caused net increases in serum total, VLDL-, IDL-, and LDL-cholesterol concentrations by 12%, 34%, 28%, and 12%, respectively; squalene by five times; and cholesterol precursor sterols by up to 60%. Fecal squalene was 15% of the dietary intake, cholesterol absorption was unchanged, fecal neutral sterols were significantly increased, whereas, in contrast to the precursor sterols, the increase in cholesterol synthesis was insignificant. LDL apolipoprotein B was increased by 14% with unchanged removal but enhanced transport of LDL apolipoprotein B. A negative correlation between the changes in LDL apolipoprotein B removal and LDL cholesterol suggests that LDL receptor activity was down-regulated, allowing more of the LDL precursor lipoproteins to be converted to LDL. A subsequent 6-wk period on 0.5 g squalene/d normalized serum sterols.