Background: Nitric oxide (NO) can be both pro- and anti-apoptotic in various cell types, including macrophages. This apparent paradox may result from the actions of NO-related species generated in the microenvironment of the cell, for example the formation of peroxynitrite (ONOO-). In this study we have examined the ability of NO and ONOO- to evoke apoptosis in human monocyte-derived macrophages (?), and investigated whether preconditioning by cyclic guanosine monophosphate (cGMP) is able to limit apoptosis in this cell type. Methods: Characterisation of the NO-related species generated by (Z)-1- [2-(2-aminoethyl)-N- (2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA/NO) and 1,2,3,4-oxatriazolium, 5-amino- 3-(3,4-dichlorophenyl)-, chloride (GEA-3162) was performed by electrochemistry using an isolated NO electrode and electron paramagnetic resonance (EPR) spectrometry. Mononuclear cells were isolated from peripheral blood of healthy volunteers and cultured to allow differentiation into ?. Resultant ? were treated for 24 h with DETA/NO (100 . 1000 ?M) or GEA-3162 (10 . 300 ?M) in the presence or absence of BAY 41.2272 (1 ?M), isobutylmethylxanthine (IBMX; 1 ?M), 1H- [1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ; 20 ?M) or 8-bromo-cGMP (1 mM). Apoptosis in ? was assessed by flow cytometric analysis of annexin V binding in combination with propidium iodide staining. Results: Electrochemistry and EPR revealed that DETA/NO liberated free NO radical, whilst GEA-3162 concomitantly released NO and O2 -, and is therefore a ONOO- generator. NO (DETA/ NO) had no effect on cell viability, but ONOO- (GEA-3162) caused a concentration-dependent induction of apoptosis in ?. Preconditioning of ? with NO in combination with the phosphodiesterase inhibitor, 3-Isobutyl-1-methylxanthine (IBMX), or the NO-independent stimulator of soluble guanylate cyclase, BAY 41.2272, significantly attenuated ONOO--induced apoptosis in a cGMP-dependent manner. Conclusion: These results demonstrate disparities between the ability of NO and ONOO- to induce apoptosis in human ?. Furthermore, this study provides evidence for a novel cGMPdependent pre-conditioning mechanism to limit ONOO--induced apoptosis in human ?. ? 2009 Shaw et al; licensee BioMed Central Ltd.