The enhanced nitric oxide (NO) and prostaglandin (PG) generation of activated macrophages is controlled by glucocorticoid-sensitive inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), respectively. Negative feedback regulation of iNOS expression by the products of both pathways has been suggested, but their effects on COX-2 expression have not been examined. We hae investigated the effect of E- and l-series prostaglandins that activate adenylate cyclase (AC), forskolin (a direct activator of AC), and other agents that influence the cyclicAMP/cyclicGMP systems on the ability of E. coli endotoxin (lipopolysaccharide, LPS) to induce iNOS and COX-2 in the murine macrophage cell line J774. After a 2-hr pretreatment before adding endotoxin, PGE2, PGI2, forskolin, IBMX (isobutylmethylxanthine, a cyclicAMP/cyclicGMP phosphodiesterase inhibitor), 8-bromo cyclicAMP, and arachidonic acid itself all inhibited the expression of both iNOS and COX-2 (as shown by Western blotting) and reduced NO release and COX activity, whereas PGF2 alpha and 8-bromo cyclic GMP were only weakly effective. The effects of PGE2, PGI2, and forskolin were enhanced by cotreatment with IBMX. The suppression of LPS-induced iNOS induction by PGE2 was functionally significant, in that it protected against the mild cytotoxicity of the NO generated in response to endotoxin. These results provide the first direct evidence for the feedback regulatory suppression of COX-2 induction by a PG-driven cAMP-mediated process, and show that the modulation of iNOS and COX-2 induction shares common features. They also suggest that such modulation is normally held in check by high phosphodiesterase activity within these cells.