Metabolism of 32-oxo-24, 25-dihydrolanosterols (3β-hydroxylanost-8-en-32-al (4, Δ⁸-CHO) and 3β-hydroxylanost-7-en-32-al (5, Δ⁷-CHO)) was studied in a reconstituted system consisting of rat liver partially purified cytochrome P-450, which catalyzes lanosterol 14-demethylation (P-450_14DM), and reduced nicotineamide adenine dinucleotide phosphate (NADPH)-cytochrome P-450 reductase. The reconstituted system converted Δ⁸-CHO (4) to 4, 4-dimethyl-5α-cholesta-8, 14-dien-3β-ol (2, 8, 14-Diene), which corresponds to the 14-deformylated product. Δ⁷-CHO (5), the isomer of Δ⁸-CHO (4), was not converted to the corresponding 14-deformylated product. The apparent K_m value of cytochrome P-450_14DM for Δ⁸-CHO (4) was about 1/20 of that for 24, 25-dihydrolanosterol (1, DHL). The metabolism of Δ⁸-CHO (4) was inhibited by 7-oxo-24, 25-dihydrolanosterol (6, 7-oxo-DHL), which is a potent inhibitor of cholesterol biosynthesis from lanosterol or DHL (1). However, the metabolism of Δ⁸-CHO (4) was less inhibited by 7-oxo-DHL (6) than that of DHL (1).