Nitric oxide (NO(.-)) contributes to vascular collapse in septic shock and regulates inflammation. Here, we demonstrate in lipopolysaccharide (LPS)-stimulated human THP-1 cells and monocytes that NO(.-) regulates interleukin (IL)-8 and tumor necrosis factor alpha (TNF-alpha) by distinct mechanisms. Dibutyryl-cyclic guanosine 5'-monophosphate (cGMP) failed to simulate NO(.-)-induced increases in TNF-alpha or IL-8 production. In contrast, dibutyryl-cyclic adenosine monophosphate blocked NO(.-)-induced production of TNF-alpha (P=0.009) but not IL-8. NO(.-) increased IL-8 (5.7-fold at 4 h; P=0.04) and TNF-alpha mRNA levels (2.2-fold at 4 h; P=0.037). However, nuclear run-on assays demonstrated that IL-8 transcription was slightly decreased by NO(.-) (P=0.08), and TNF-alpha was increased (P=0.012). Likewise, NO(.-) had no effect on IL-8 promoter activity (P=0.84) as measured by reporter gene assay. In THP-1 cells and human primary monocytes treated with actinomycin D, NO(.-) had no effect on TNF-alpha mRNA stability (P>0.3 for both cell types) but significantly stabilized IL-8 mRNA (P=0.001 for both cell types). Because of its role in mRNA stabilization, the p38 mitogen-activated protein kinase (MAPK) pathway was examined and found to be activated by NO(.-) in LPS-treated THP-1 cells and human monocytes. Further, SB202190, a p38 MAPK inhibitor, was shown to block NO(.-)-induced stabilization of IL-8 mRNA (P<0.02 for both cell types). Thus, NO(.-) regulates IL-8 but not TNF-alpha post-transcriptionally. IL-8 mRNA stabilization by NO(.-) is independent of cGMP and at least partially dependent on p38 MAPK activation.