Meso-dihydroguaiaretic acid (MDGA) is a medicinal herbal product isolated from the aerial parts of Saururus chinensis that inhibits the cyclooxygenase-2 (COX-2)-dependent phase of prostaglandin $D_2;(PGD_2)$ generation in bone marrow-derived mast cells (BMMC) ($IC_{50};9.8;{mu}M$). However, this compound did not inhibit COX-2 protein expression in BMMC at concentrations up to $30;{mu}M$, indicating that MDGA directly inhibits COX-2 activity. In addition, this compound consistently inhibited the production of leukotriene $C_4;(IC_{50};1.3;{mu}M)$. These results demonstrate that MDGA inhibits both COX-2 and 5-lipoxygenase. Furthermore, this compound strongly inhibited the degranulation reaction in BMMC ($IC_{50};11.4;{mu}M)$. Therefore, this compound might provide a basis for novel anti-inflammatory drug development.