The rates of incorporation of 14C-mevalonate into cholesterol, lanosterol, squalene and other intermediary metabolites such as allyl pyrophosphates, in rat liver micro-somes and soluble cell components were studied. In this way the effects of various steroids and detergents added to the incubation medium on the sequential metabolic pathway of cholesterol synthesis were observed. On addition of steroids, 14C-incorporation from mevalonate into the total non-saponifiable lipids was generally slightly inhibited and that into cholesterol was markedly decreased, while incorporation into lanosterol was increased. Therefore steroids may preferentially inhibit cholesterol synthesis after mevalonate formation at the step between lanosterol and cholesterol. The effects of the steroids tested decreased in the following order: progesterone, testosterone > androsterone, dehydro-epiandrosterone, estradiol-17β> hydrocortisone, cortisone. The addition of some detergents to the incubation medium seemed to inhibit not only cholesterol formation from lanosterol but also cyclization of squalene to lanosterol, though their effect on the latter was slight. Sterols derived from 14C-mevalonate may be fixed in microsomal pellets, mainly in the free form. However, their release from the microsomes on sonication was increased by incubation of the microsomes with progesterone, possibly due to the detergent like action of the steroid. The effect of progesterone seemed to be reversible, since the rate of cholesterol synthesis was restored by addition of lecithin alone or of lecithin-cholesterol micelles even when progesterone was also added. The inhibition of cholesterol synthesis by steroids seems to be closely related to the function of the microsomal membrane.