3-Hydroxy-3-methylglutaric acid (HMGA) is a competitive inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoAR) and strongly reduces cholesterol biosynthesis both in vitro and in vivo. Since the effects of this compound on biliary lipid composition are hitherto unknown, we have investigated whether it prevents dietary cholesterol induction of saturated bile in the hamster. Female Golden Syrian hamsters have been divided into four groups and treated for 10 weeks as follows: I) Standard diet, containing 0.8 mg cholesterol/g food; II) Standard diet plus HMGA (100 mg/kg b.w./day per os); III) Lithogenic diet containing 2.4 mg cholesterol/g food; IV) Lithogenic diet plus HMGA as above. The results indicate that HMGA is effective in reducing both bile cholesterol supersaturation and hypercholesterolemia. Inhibition of hepatic cholesterogenesis at the level of mevalonate synthesis by HMGCoAR and reduction of intestinal cholesterol absorption may be responsible for these effects.