Background: Nitric oxide (NO) is an important mediator of inflammatory processes, including macrophage-mediated cellular host defense, and is found to be increased in peritonitis. The ability of human mononuclear cells to contribute to the NO production by expression of active inducible NO synthase (iNOS) is still discussed controversely. Aims: This study was designed to investigate the influence of prostacyclin receptor (IP receptor) activation on iNOS expression and NO formation in human peripheral blood monocytes. Method and Results: Using reverse transcriptase-polymerase chain reaction, we demonstrated that human monocytes express high levels of IP receptor mRNA. Stimulation of monocytes with the IP receptor selective agonist cicaprost (100 n M) significantly induced cellular cyclic adenosine monophosphate formation, indicating functional coupling of the receptor to G<sub>s</sub>. Treatment of cells with lipopolysaccharide (LPS)/interferon gamma (IFN-γ) further enhanced the IP receptor mRNA expression 2.7 ± 0.1-fold above basal levels (n = 6). Analysis of iNOS expression revealed barely detectable mRNA levels in unstimulated monocytes which were increased 3.75 ± 0.3-fold (n = 5) after costimulation with 1 µg/ml LPS and 250 U/ml INF-γ for 16 h. Further increases of iNOS mRNA expression (9.4 ± 0.9-fold above basal, n = 5) were obtained, if the monocytes were costimulated with 1 µg/ml LPS, 250 U/ml INF-γ, and 100 n M cicaprost for 16 h. Measurement of the NO generation correlated with the polymerase chain reaction data: treatment of cells with 1 µg/ml LPS plus 250 U/ml INF-γ increased the NO<sub>2</sub> production to 2.6 µ M, being above the basal level of 2.0 µ M, as determined in the cell culture medium. Additional treatment with 100 n M cicaprost further significantly increased the NO<sub>2</sub> production to 3.43 µ M. Conclusions: An IP receptor mediated increase in cyclic adenosine monophosphate formation plays an important role in enhancing LPS/IFN-γ-induced iNOS expression in human monocytes/macrophages and may, therefore, contribute to the increased production of NO during peritonitis.