Cytosolic HMG-CoA synthase and microsomal 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase catalyze two sequential steps in the mevalonate pathway. Both enzymes are negatively regulated by cholesterol. Cytosolic HMG-CoA synthase is responsible for the generation of HMG-CoA from acetyl-CoA and acetoacetyl-CoA). We have developed a new method to determine HMG-CoA synthase activity. In this assay, HMG-CoA is formed from acetoacetyl-CoA and labeled acetyl-CoA. The HMG-CoA product is isolated from the reaction mixture by means of reversed-phase ion-pair chromatography. The recovery of the product was always greater than 90%. The average within-batch coefficient of variation for HMG-CoA synthase activity was 5.1%. Using the new assay, we demonstrate that Lifibrol (K12.148), a new hypolipidemic compound, inhibits HMG-CoA synthase. Because our assay is accurate and precise it may become useful in future studies on the regulation and the pharmacological modulation of cytosolic HMG-CoA synthase.