Measurement of mevalonic acid (MVA) concentrations in plasma or 24-h urine samples is shown to be useful in studies of the regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and cholesterol synthesis. Plasma MVA concentrations, measured either at 7-9 a.m. after an overnight fast, or throughout the 24-h cycle, were compared with cholesterol synthesis rates that were measured by the sterol balance method: plasma MVA concentrations were directly related to the rate of whole body cholesterol synthesis (r = 0.972; p less than 0.001; n = 18) over a tenfold range of cholesterol synthesis rates. Moreover, hourly examination of MVA concentrations throughout the day demonstrated that interventions such as fasting or cholesterol feeding cause suppression of the postmidnight diurnal rise in plasma MVA concentrations, with little change in the base-line of the rhythm. Thus, the daily rise and fall of plasma MVA appears to reflect changes in tissues and organs, such as the liver and intestine, that are known to be most sensitive to regulation by fasting or by dietary cholesterol. The hypothesis that short-term regulation of HMG-CoA reductase in tissues is quickly reflected by corresponding variations in plasma MVA was tested by using a specific inhibitor of HMG-CoA reductase, mevinolin, to block MVA synthesis. Mevinolin caused a dose-dependent lowering of plasma MVA after a single dose; and in patients who received the drug twice a day for 4 wk, it decreased 24-h urinary MVA output. Significant lowering of plasma cholesterol was achieved through administration of mevinolin at doses that only moderately limit MVA production.