PTMC, one of the choleretic, as is well known, shows a considerable serum cholesterollowering activity. In order to study this effect, th is paper will descr ible results in the effect of PTMC on hepat ic cholesterol biosynthesis and fecal excret ion of bile acids and sterols. Male mice weighing about 20g were injected for 12 consecutive days wi th PTMC (100mg/ kg/day). One hour p r io r to sacrifice on the 16 days, the animals were injected with 5 Vc of acetate-l-~4C of 0.12t~c of dl-mevalonic acid-2-14C per 20g of body weight. The liver were excised. Cholesterol was isolated as the digitonide and the fat ty acids extracted with petroleum ether, after alkaline digest was acidified. Radioactivity measurements were carried out in gas flow counter. Male ra ts weighing about 90g were used. The two-week exper imental period was divided into three groups: Unt rea ted control, cholesterol fed (100mg/day) and cholesterol f e d § (100mg/kg/day) t rea ted group. Feces were analyzed for bile acids and sterols by par t i t ion chromatography. PTMC resulted in an inhibit ion of cholesterol and fat ty acid synthesis from acetate-ln4C. Futher-more the incorporat ion of mevalonic acid-2-~*C into cholesterol was inhibited. PTMC reduced hepatic and serum cholesterol. The adminis t ra t ion of PTMC in cholesterol fed ra ts for 14 days is followed by an increase in the fecal excre t ion of bile acid, while there was no change in fecal excret ion of sterols. It is concluded tha t the mechanism of action of PTMC in lowering serum cholesterol is regard to an inhibi t ion on the biosynthesis of cholesterol and the increased el imination of bile acids with the feces.