We prepared a number of N-phenethyltetrahydroisoquinolines structurally related to protoberberines. They were tested for activity against bacteria, fungi, and human leukemia HL-60 cells and also for inhibition of biosynthesis: ergosterol in yeasts and cholesterol in human cells. In the latter assay panel, several of the compounds were distinguished by a strong and selective inhibition of 7-dehydrocholesterol reductase (7-DHCR, EC 1.3.1.21), an enzyme responsible for the conversion of 7-dehydrocholesterol to cholesterol in the last step of cholesterol biosynthesis. In a whole-cell assay, the most active compound 5f showed a much stronger inhibition of overall cholesterol biosynthesis (IC(50) 2.3 nM) than BM 15.766 (IC(50) 500 nM), presently the most selective known inhibitor of 7-DHCR. Since a defect of 7-dehydrocholesterol reductase is associated with Smith-Lemli-Opitz syndrome (SLOS), the potent and selective inhibitors reported here will enable more detailed investigation of the pathogenesis of SLOS.