Mevinolin (MK-803) is a potent inhibitor of HMG-CoA reductase. After a placebo run-in period, mevinolin 5,15 or 50 mg, or placebo was given twice daily for 7-11 days under double -blind conditions ot 4 groups of 6 normocholesterolemic male volunteers. After 7 days, mean serum cholesterol fell 14%, 25% and 24% on 5, 15 and 50 mg, respectively, which was significantly greater than the fall on placebo (4%) in the case of the two higher doses (P less than 0.01). Serum triglycerides did not change significantly. Mevinolin was generally well-tolerated and there were no serious adverse effects.