We examined whether octyl methoxycinnamate (OMC), a UV-filter, is metabolically activated by liver microsomes of rats and humans in respect to endocrine-disrupting action. OMC itself showed no agonistic activity towards estrogen receptor (ER) or aryl hydrocarbon receptor (AhR), and no antagonistic activity towards ER or androgen receptor (AR). The hydrolysis product, 4-methoxycinnamic acid (4-MCA), was also inactive in all the assays. In contrast, the desmethylated product, octyl hydroxycinnamate (OHC), exhibited agonistic activities towards ERα, ERβ and AhR. Importantly, when OMC was incubated with rat liver microsomes in the presence of NADPH, the major product was 4-MCA, and OHC was not formed at all. 4-MCA was also produced as the main metabolite of OMC by pooled human liver and small-intestinal microsomes. The OMC-hydrolyzing activity was higher in small-intestinal microsomes than in liver microsomes of both rats and humans, and was higher in humans than in rats. Therefore, OMC hydrolysis appears to be mainly catalyzed by small-intestinal carboxylesterase 2 isoforms, and partly by liver carboxylesterase 1 isoforms. We confirmed that OMC was hydrolyzed by human recombinant carboxylesterase isozymes, CES1b, CES1c and CES2. Our results indicate that OMC is not metabolically activated to OHC in humans, but is mainly hydrolyzed to inactive 4-MCA, suggesting that it is unlikely to pose a risk of human health in terms of endocrine-disrupting activity.