The inhibitory effects of beta-sitosterol on intestinal cholesterol absorption were studied by means of a dual isotope plasma ratio method (in vivo), which is a new technique for the measurement of cholesterol absorption, as well as a ligated-loop method (in situ). The results obtained were as follows: 1. The absorption of beta-sitosterol itself was significantly less than cholesterol. Cholesterol was selectively absorbed from rat intestine. 2. When 100 to 1,000 microgram of beta-sitosterol were added to the dose solution containing 10 microgram of cholesterol, cholesterol absorption by the in vivo experiment decreased with the increae of additional beta-sitosterol. 3. A similar inhibitory effect of beta-sitosterol was observed by the in situ ligated-loop method. These results suggest that beta-sitosterol actually inhibits cholesterol absorption in the physiological state of an animal.