Determination of a 24-hour lipoprotein lipid profile on a mixed diet showed a typical increase in VLDL (includes chylomicrons) lipids following food intake in normal subjects. The change was less marked or absent in patients with jejuno-ileal by-pass and after cholestyramine treatment of a case with post-ileal-resection diarrhoea. Triglycerides of other lipoproteins did not fluctuate very much but free and esterified cholesterol of LDL and HDL showed a significant diurnal variation: lowest values were seen early in the morning followed by an increase before breakfast, with the highest levels during the afternoon in most cases. No clearcut precursor-product relationship could be seen between VLDL and LDL or HDL, yet the rhythmic changes in the VLDL levels and/or in cholesterol synthesis may be causally associated with the diurnal variation of cholesterol in LDL and HDL.