Phorbol-12-myristate- 13-acetate (PMA) has been shown to induce hypertrophy of cardiac myocytes. The prostaglandin endoperoxide H synthase isoform 2 (cyclooxygenase-2, COX-2) has been associated with enhanced growth and/or proliferation of several types of cells. Thus we studied whether PMA induces COX-2 and prostanoid products PGE₂ and PGF₂ in neonatal ventricular myocytes and whether endogenous COX-2 products participate in their growth. In addition, we examined whether PMA affects interleukin-1 (IL-1) stimulation of COX-2 and PGE₂ production. PMA (0.1 µmol/l) stimulated growth, as indicated by a 1.6-fold increase in [³H]leucine incorporation. PMA increased COX-2 protein levels 2.8-fold, PGE₂ 3.7-fold, and PGF₂ 2.9-fold. Inhibition of either p38 kinase or protein kinase C (PKC) prevented PMA-stimulated COX-2. Inhibition of COX-2 with either indomethacin or NS-398 had no effect on PMA-stimulated [³H]leucine incorporation. Exogenous administration of PGF₂, but not PGE₂, stimulated protein synthesis. Treatment with IL-1 (5 ng/ml) increased COX-2 protein levels 42-fold, whereas cotreatment with IL-1 and PMA stimulated COX-2 protein only 32-fold. IL-1 did not affect control or PMA-stimulated protein synthesis. These findings indicate that: 1) PMA, acting through PKC and p38 kinase, enhances COX-2 expression, but chronic treatment with PMA partially inhibits IL-1 stimulation of COX-2; and 2) exogenous PGF₂ is involved in neonatal ventricular myocyte growth but endogenous COX-2 products are not.