Pairs of littermate young dogs were fed for 7 or 11 days on a diet low in cholesterol, followed for a similar period by a diet containing 1 per cent. cholesterol. They were killed 20 hr. after the last feeding. Slices of liver and other tissues were then incubated for 3 hr. at 37^° C. in a buffered medium containing ¹⁴C-carboxyl-labelled acetate. Cholesterol and fatty acids were isolated from the incubation mixture for radio-active assay. Liver tissue of normal dogs regenerated and synthesised cholesterol more rapidly than any other tissue, but regeneration and synthesis were depressed in animals given cholesterol; no such depression was observed with skin or intestinal mucosa. The influence of dietary cholesterol on synthesis in the liver was specific and there was no effect on the rate of fatty acid synthesis. G. A. Garton.