A quantitative structure-activity relationship study has been made of some 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors. The HMG-CoA reductase inhibition activities of mevinolin analogs and 6-substituted 4-hydroxypyran-2 ones have been mostly found to be significantly correlated with the molecular size of substituents. In one case, however, the inhibition potency was found to be related to the hydrophobicity of molecules. These findings led us to suggest that the enzyme HMG-CoA reductase possesses an active site which is involved in dispersion interaction and another site which is involved in hydrophobic interaction with inhibitor molecules, depending upon the proper orientation of the latter towards these sites. Furthermore, the results indicate that both active sites possess limited steric bulk tolerance.