A mitogen-inducible prostaglandin G/H synthase (PGHS-2 or cyclooxygenase-2) has recently been cloned from chicken and mouse fibroblasts. This protein is distinct from classic prostaglandin G/H synthase (PGHS-1 or cyclooxygenase-1) but has a similar enzymatic activity. Because PGHS-1 is a rate-limiting enzyme in the synthesis of prostaglandins, PGHS-2 may also play an important role in prostaglandin production. To examine whether PGHS-2 is induced by phorbol ester in mast cells, we studied mRNA expression of PGHS-2 and also measured prostaglandin D2 (PGD2) production when canine mastocytoma cells were incubated with phorbol myristate acetate (PMA). PGHS-2 mRNA was induced by PMA, with a maximal induction after 4 h of incubation with 10 nM PMA. There was concentration-dependent production of PGD2 after incubation with PMA. In contrast, PGHS-1 mRNA was expressed in resting cells, and the expression of PGHS-1 mRNA was down-regulated by PMA. Dexamethasone inhibited PMA-induced mRNA expression of PGHS-2 and PGD2 production. Aspirin had no effect on mRNA expression of PGHS-2 but inhibited PGD2 production. In conclusion, PGHS-2 is induced by phorbol ester in canine mast cells. We speculate that PGHS-2 may be important in airway inflammation in which mast cells are activated.