The therapeutic effect of a single sustained release tablet of bezafibrate (Cedur retard, 400 mg) in primary hypercholesterolemia type IIa and type IIb was investigated in a placebo-controlled randomised study comparing the efficacy of morning vs. evening intake. The decrease in total cholesterol with the morning intake was 18.5% vs. 16.5% for the evening intake (n.s.). HDL-cholesterol increased more in patients taking bezafibrate retard at morning (29.6% vs. 22.4%, p less than 0.05). Bezafibrate was well tolerated. Animal experiments and precursor studies of cholesterol synthesis in man indicate peak activity of HMG-CoA reductase between 1 a.m. and 3 a.m. The data suggest that with normal eating habits during day time other modes of action of bezafibrate besides HMG-CoA reductase inhibition such as reduction of VLDL-synthesis in the liver and an increased fractional catabolic rate, could contribute to the therapeutic effect.