AbstractAn investigation was undertaken to study the effect of sex hormones on cholesterol biosynthesis by using the albino rats of the University of Southern California strain fed on three different kinds of diets: (a) Control diet, containing 15% cottonseed oil (I); (b) Diet (I) containing 1% cholesterol (II); (c) Fat‐free diet (III).By liver slice technique, the results showed the following:(1)In rats fed with control diet (I), the male had a much higher rate of cholesterol biosynthesis than the female; and the cholesterol biosynthesis in both male and female rats was decreased by gonadectomy as compared with the intact male and female animals on the same diet.(2)In rats fed with diet (II) or (III), the cholesterol biosynthesis was greatly decreased, due to the feeding of cholesterol‐containing or fat‐free diet so that gonadectomy caused no more distinct effect on the cholesterol biosynthesis.(3)The cholesterol biosynthesis was greatly enhanced by sex hormone administration in rats fed the three kinds of different diets.The effect of sex hormones to increase the cholesterol biosynthesis in liver is probably their secondary influence caused by their primary influence to reduce the liver cholesterol by different mecha