Activation of the macrophage cell line RAW 264.7 with LPS and IFN-γ induces apoptosis through the synthesis of high concentrations of NO due to the expression of NO synthase-2. In addition to NO, activated macrophages release other molecules involved in the inflammatory response, such as reactive oxygen intermediates and PGs. Treatment of macrophages with cyclopentenone PGs, which are synthesized late in the inflammatory onset, exerted a negative regulation on cell activation by impairing the expression of genes involved in host defense, among them NO synthase-2. However, despite the attenuation of NO synthesis, the percentage of apoptotic cells increased with respect to activated cells in the absence of cyclopentenone PGs. Analysis of the mechanisms by which these PGs enhanced apoptosis suggested a potentiation of superoxide anion synthesis that reacted with NO, leading to the formation of higher concentrations of peroxynitrite, a more reactive and proapoptotic molecule than the precursors. The effect of the cyclopentenone 15-deoxy-Δ 12, 14-PGJ 2 on superoxide synthesis was dependent on p38 mitogen-activated protein kinase activity, but was independent of the interaction with peroxisomal proliferator-activated receptor γ. The potentiation of apoptosis induced by cyclopentenone PGs involved an increase in the release of cytochrome c from the mitochondria to the cytosol and in the nitration of this protein. These results suggest a role for cyclopentenone PGs in the resolution of inflammation by inducing apoptosis of activated cells.