HMG-CoA reductase inhibitors, such as pravastatin, are widely used as lipid lowering drugs in hypercholesterolemia. Pravastatin does not only reduce the atherogenic low density lipoprotein (LDL)-cholesterol, but is also increasing high density lipoprotein (HDL)-cholesterol. However, the mechanism leading to an increase of HDL are unclear. Therefore, the effects of pravastatin on the in vivo kinetics of apolipoprotein (apo) A-I were studied in six normolipidemic subjects and in a patient with coronary artery disease (CAD) utilizing stable isotope tracer techniques. Two turnover studies were performed. The first turnover study was carried out before any drug treatment, the second study after 6 weeks of 40 mg pravastatin/day. Three times deuterium labeled L-leucine (3D-leucine) was given as a primed bolus constant infusion (bolus: 1340 microg/kg; infusion: 22 microg/kg per h), and tracer uptake into HDL apoA-I was determined by gas chromatography (GC)-mass-spectrometry (MS). In the healthy subjects HDL-cholesterol increased by 13% and apoA-I increased by 12% under pravastatin treatment. The HDL in the CAD patient decreased by 3% and apoA-I increased by 2%. Prior to drug treatment the mean apoA-I fractional synthetic rate (FSR) was 0.194 per day (S.D. +/- 0.02) and apoA-I production rate (PR) was 10.8 mg/kg per day (S.D. +/- 2.1). The CAD patient had a FSR of 0.219 per day and a PR of 10.6 mg/kg per day. After treatment with pravastatin the mean apoA-I FSR was 0.204 per day (S.D. +/- 0.02) and apoA-I PR was 12.5 mg/kg per day (S.D. +/- 1.5) in the healthy subjects. Despite only minor changes of HDL and apoA-I in the CAD patient, there were significant changes of FSR (0.267 per day) and PR (13.1 mg/kg per day) with pravastatin treatment. The in vivo kinetic data demonstrate an increased FSR of apoA-I. The increase in apoA-I is due to an increased PR of apoA-I. This study demonstrates increased production of HDL apoA-I as the metabolic cause of the increase in HDL and apoA-I levels under inhibition of HMG-CoA reductase in man.