We describe an indirect method for assessing variation in 3-hydroxy-3-methylglutaryl-coenzyme A reductase (NADPH) activity in liver tissue. 3-Hydroxy-3-methyglutaryl-CoA and mevalonate concentrations in the tissue homogenate are estimated in terms of absorbances and the ratio between the two is taken as an index of activity of the enzyme, which catalyzes the conversion of 3-hydroxy-3-methylglutaryl-CoZ to mevalonate. The ratio increases under conditions in which activity of this enzyme in liver reportedly decreases (e.g., fasting, cholesterol feeding) and decreases under conditions in which the activity of this enzyme reportedly increases (e.g., Triton injection, phenobarbital treatment). We assessed the magnitude of hepatic cholesterogenesis in intact male albino rats by using [1,2-14C] acetate, in fed controls, fasted, cholesterol-fed, Triton-treated, and phenobarbital-treated. The ratio, 3-hydroxy-3-methylglutaryl-CoA/mevalonate was estimated in liver tissue for all the above groups. The method is simple, reproducible, and does not require any unusual chemicals.