A series of 20 mevalonic acid analogs was synthesized and tested for their ability to inhibit cholesterol biosynthesis from [2-14C]-mevalonate in rat liver homogenates. Removal of the 5-hydroxyl group from mevalonic acid produced an active inhibitor, 3-hydroxy-3-methylpentanoic acid. Removal of the 3-hydroxyl group, addition of an aromatic group in the 3-position, or insertion of a double bond reduced inhibitory activity. Compounds with an aromatic group or halide on the 5-position were active inhibitors. The most active inhibitor was 5-phenylpentanoic acid, with 50% inhibition at 0.064 mM.