Research initiated by the discovery that 4, 4, 10Beta-trimethyl-transdecal-3Beta-ol (TMD) is an effective inhibitor of cholesterol biosynthesis at the oxidosqualene cyclase stage will be continued. This finding has enabled an ongoing investigation of the biochemistry of squalene 2,3(Sapproximately);22(Sapproximately),23-dioxide and related dioxygenated triterpenoids and steroids. Of particular potential significance is the finding that 24(Sapproximately),25-epoxycholesterol is formed in the normal course of sterol biosynthesis by the enzymes in rat liver homogenate. It will be determined whether this epoxide occurs in intact tissue and, if so, what its biochemical properties and metabolic fate are. Greater understanding of the natural mechanisms of cholesterol formation and regulation may result from this research, and such understanding will eventually be important in achieving control of atherosclerosis. The inhibition of cholesterol biosynthesis by TMD has also proved to be valuable in collaborative studies of cell replication, and these investigations will continue. The effect of substances structurally related to TMD on cholesterol biosynthesis will be studied in order to elucidate the structural requirements for inhibition and to develop additional inhibitors which will be useful as research tools. TMD and related inhibitors of oxidosqualene cyclase will also be used to try to gain insight into the structure and mechanism of this important enzyme through efforts at purification by affinity chromatography and at affinity labelling of the active side.