The time course of the changes in serum lipid and lipoprotein levels have been studied in 100 patients undergoing coronary artery bypass surgery. Marked decreases occur in the levels of cholesterol (-45%), HDL-cholesterol (-35%), LDL-cholesterol (-53%), Apo A1 (-43%) and Apo B (-43%) on the third day after the operation. In the control group the values gradually returned to preoperative levels which are reached after three months. Three months after the intervention, in the pravastatin-treated group (20 mg/day) the values of total cholesterol (-20%), LDL-cholesterol (-28%) and Apo B (-22%), remained significantly lower than in the control group (p 0.0001) and were lower over the whole duration of the study. The postoperative time course of HDL-cholesterol and Apo A1, however, was not significantly influenced by the administration of pravastatin.