When 25-hydroxycholesterol or 7-ketocholesterol was fed to mice with the diet, growth was suppressed and mature mice lost weight. The effect of the 7-ketone upon body weight was effectively counteracted by cholesterol whereas cholestanol and beta-sitosterol were ineffective. Growth repression due to 25-hydroxycholesterol was only partially relieved by cholesterol. The effects of 25-hydroxycholesterol and 7-ketocholesterol upon body weight were related to an apparent effect upon appetite. However the sterols were not unpalatable since diets containing them were not rejected in favor of control diet. Intestinal sterol synthesis was inhibited soon after the administration of dietary 7-ketocholesterol or 25-hydroxycholesterol but inhibition decreased with prolonged feeding. When fed by gavage, the sterols suppressed intestinal sterol synthesis as soon as 2 h after administration. In contrast, cholesterol administered by gavage did not affect intestinal sterol synthesis during a 24 h test period. When fed with the diet 25-hydroxycholesterol and 7-ketocholesterol did not depress hepatic cholesterol synthesis beyond the low levels found in pair-fed controls. Inhibition of intestinal sterol synthesis was accompanied by a decrease in the concentration of cholesterol in the intestinal mucosa and, usually, by a drop in the molar ratio of cholesterol to phospholipids.