Four new coumarins (2',3'-dihydroxyphellopterin, E-5-methoxytrichoclin acetate, Z-5-methoxytrichoclin acetate, and E-5-methoxytrichoclin) and three known coumarins (byakangelicol, byakangelicin, and Z-5-methoxytrichoclin) were produced by liver microsomes from rats pre-treated with sodium phenobarbital. The chemical structures were elucidated on the basis of their spectroscopic data. The inhibitory activities of nitric oxide (NO) production in lipopolysaccharide-activated macrophage-like cell line RAW264.7 were tested. The main biotransformation product, byakangelicin, showed inhibitory activities of NO production with the IC₅₀ value of 217.83 μM, whereas the parent compound phellopterin showed cytotoxic effect on RAW264.7 cell at the concentration from 40 to 400 μM.