The effect of SMP-500, a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on serum cholesterol levels was investigated in hyperlipidemic hamsters whose condition had been preestablished by diet. SMP-500 reduced the total serum cholesterol level in a dose-dependent manner. SMP-500 also reduced the hepatic free cholesterol content and markedly reduced the esterified cholesterol content compared with the control group. Interestingly, SMP-500 at a dose of 30 mg/kg increased LDL clearance in vivo. As SMP-500 at this dose potently lowered the total serum cholesterol level, the increased LDL clearance was identified as another mechanism for the cholesterol-lowering effect of SMP-500. However, unlike HMG CoA reductase inhibitors, SMP-500 did not affect cholesterol biosynthesis in HepG2 cells. Therefore the etiology of the increased LDL clearance is not yet clear, but the reduced hepatic free cholesterol may play an important role in this process. These results suggest that the cholesterol-lowering effect of SMP-500 is due, not only to the inhibition of ACAT, but also to the increase in cholesterol clearance from the blood. This finding supports the therapeutic potential of SMP-500 for the treatment of human hypercholesterolemia.