Sesamin, a lignan in sesame seeds and sesame seed oil, and capsaicin, the pungent principle of hot red pepper, affect lipid metabolism. Sesamin specifically inhibits delta5 desaturase activity in the Mortierella alpina fungus and rat liver microsomes, however, the effects of sesamin and capsaicin on mRNA expressions of delta6 and delta5 desaturases are not still clear. In this study, we investigated the effects of sesamin and capsaicin on the desaturation indexes of delta6 [(gamma-linolenic acid+dihomo-gamma-linolenic acid)/linolenic acid, (GLA+DGLA)/LA] and delta5 (arachidonic acid/DGLA, AA/DGLA) and mRNA expressions of delta6 and delta5 desaturases in rat primary cultured hepatocytes. To measure the mRNA expressions of delta6 and delta5 desaturase, hepatocytes were cultured in the presence of sesamin or capsaicin for 24 h. To investigate the delta6 or delta5 desaturation index, hepatocytes were cultured in the presence of LA or DGLA, respectively, with sesamin or capsaicin for 24 h. The fatty acid composition of the cells was measured by GLC. The mRNA expressions of delta6 and delta5 desaturases were detected by real time quantitative RT-PCR. Sesamin and capsaicin had no effect on the mRNA expressions of delta6 and delta5 desaturases in rat hepatocytes. Capsaicin had no effect on both delta6 and delta5 desaturation indexes, either. On the other hand, sesamin significantly reduced the index of delta5 desaturation but not delta6 desaturation. These results suggested that sesamin reduced the delta5 desaturation index without the changing of the delta5 desaturase mRNA level.