Xanthine oxidase (XOD) and xanthine dehydrogenase (XDH) are mammalian xanthine oxidoreductases involved in the catalysis of the oxidation of hypoxanthine and xanthine to uric acid. It may be a reasonable approach to hamper XOD/XDH activity for the treatment of hyperuricemia-associated syndromes considering that excessive formation of uric acid can cause such syndromes. C. cassia Blume (Lauraceae) is traditionally employed in the treatment of gastritis, blood circulation and gout. Cinnamaldehyde derivatives from C. cassia exhibit potent in vivo XOD inhibitory activity and the mode of inhibition is investigated employing Lineweaver-Burk plot analysis. The active compounds are evaluated for their inhibitory potential to reduce serum and hepatic uric acid levels in male ICR mice, murine hyperuricemia model induced by pretreatment with the uricase inhibitor potassium oxonate. The tested compounds exhibit remarkable reductions in hepatic XDH/XOD activity in a time-dependent manner, suggesting that the observed in vivo anti-uricemic activity is partly originating from their inhibitory action on the xanthine oxidoreductases.