A novel assay method is described for rapid quantitation of reaction rate of sterol ${Delta}^7$-reductase (${Delta}^7$-SR) which catalyzes reduction of the ${Delta}^7$-double bond of sterols. Of six different organ tissues-liver, small intestine, brain, lung, kidney, and testis-. ${Delta}^7$-SR activity was detected only in liver (2.30 nmol/min/mg protein) and testis (0.11 nmol/min/mg protein). Using a newly developed method which employs diet-induced enzyme proteins and ergosterol as substrate, we assessed both kinetics ($K_m=210;{mu}M$, $V_{max}=1.93;nmol/min/mg$) and inhibition of the rat hepatic ${Delta}^7$-SR against well-studied cholesterol lowering agents such as triparanol ($IC_{50}=16;{mu}M$). 3-$eta$-[2-(diethylamino)ethoxy]androst-5-en-17-one (U18666A) ($IC_{50}=5.2;{mu}M$), and trans-1.4-bis(2-chlorobenzylaminomethyl)cyclohexane dihydrochloride (AY-9944) ($IC_{50}=0.25;{mu}M$). Of the three well-known AY-9944-sensitive cholesterogenic enzymes (i.e., ${Delta}^7$-SR, sterol ${Delta}^8$-isomerase, and sterol ${Delta}^14$-reductase). ${Delta}^7$-SR was found to be the most sensitive enzyme with a noncompetitive inhibition of this compound ($K_i=0.109;{mu}M$). Substrate specificity studies of the microsomal ${Delta}^7$-SR indicate that the relative reaction rate for 7-dehydrocholesterol and ergosterol are 5.6-fold and 1.6-fold higher than that for lathosterol. ${Delta}^7$-SR activity was also modulated by feeding rats a diet supplemented with 0.5% ergosterol (>2.6-fold) in addition to 5.0% cholestyramine plus 0.1% lovastatin ($simeq$5.0-fold). Finally, microsomal ${Delta}^7$-SR was solubilized by 1.5% 3-[3-(cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) and enriched on PEG (0~10%) precipitation, which should be suitable for further purification of the enzyme.